Cells sentence example
Every second, millions of cells die in your body and millions are born.
In every cell of your body except your red blood cells exists a copy of your DNA.
Each of those new cells has a new copy of your DNA.
The body-cavity is largely occupied by processes from the large muscle cells of the skin.
Hansen counted the number of yeast cells suspended in a drop of liquid diluted with sterilized water.Advertisement
These consist of enormous cells with nuclei so large as to be in some cases just visible to the naked eye.
The flasks were then well shaken, and the yeast cell or cells settled to the bottom, and gave rise to a separate yeast speck.
The preference exhibited by yeast cells for sugar molecules is shared by mould fungi and soluble enzymes in their fermentative actions.
Rhyn considered how he might use the demon, as he had once before. He didn't answer, pushing the door open to the cell block. Nearly all the cells were empty. "Where is everyone?" he asked.
Eventually he was able to prove that the biological doctrine of omnis cellula ecellula applies to pathological processes as well as to those of normal growth, and in his famous book on Cellular-pathologic, published at Berlin in 1858, he established what Lord Lister described as the "true and fertile doctrine that every morbid structure consists of cells which have been derived from pre-existing cells as a progeny."Advertisement
There were, however, but few prisons in France adapted for the cellular system, and the process of reconstruction has been slow, In 1898 the old Paris prisons of Grande-Roquette, Saint-Plagie and Mazas were demolished, and to replace them a large prison with 1500 cells was erected at Fresnes-ls-Rungis.
These cells are disposed in pairs, though the members of each pair are not always at the same level.
Similarly the giant cells are produced at their periphery into a number of branching processes which bear similar end-organs on their surface and in some cases terminate in them.
A battery with a sufficient number of cells is connected to these two electrodes so as to pass a current through the mercury vapour, negative electricity proceeding from the mercury cathode to the iron anode.
It is largely used for the purpose of making standard electric cells, such for example as the Weston cell.Advertisement
Then come the glandular surface (C), which is formed of smooth polished epidermis with numerous glands that secrete the fluid contents of the pitcher, and finally the detentive surface (D), of which the cells are produced into long and strong bristles which point A FIG.
The epithelial layer consists of (1) so-called " indifferent " cells secreting the perisarc or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot.
The muscular tissue consists primarily of processes from the bases of the epithelial cells, processes which are contractile in nature and may be distinctly striated.
A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The of the evolution of the ganglioncells is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements.
By means of vibrations or shocks transmitted through the - Sub water, or by displacements in the balance or position of the animal, the otoliths are caused to impinge against the bristles of the sensory cells, now on one side, now on the other, causing shocks or stimuli which are transmitted by the basal nerve-fibre to the central nervous system.Advertisement
At the fundus are placed the concrement-cells with their conspicuous otoliths (con) and the inconspicuous auditory cells, which are connected with the subumbral nerve - ring.
Sometimes the epidermis is considerably more developed by tangential division of its cells, forming a many-layered water-tissue.
And solar cells presently being developed in laboratories are doing several times better than the plants.
Here and there among the cells containing dead brood and honey an angry buzzing can sometimes be heard.
Here and there a couple of bees, by force of habit and custom cleaning out the brood cells, with efforts beyond their strength laboriously drag away a dead bee or bumblebee without knowing why they do it.Advertisement
They normally were, and if they weren't, their screaming was muted by the magic of their cells.
He gripped the bars of his cell and pressed his face against them, trying to see into the neighboring cells.
Clenching the book, he stepped into the familiar dungeon with its two small cells.
He agreed with Pasteur that the presence of living cells is essential to the transformation of sugar into alcohol, but dissented from the view that the process occurs within the cell.
Hansen pointed out that this was by no means the case, for it is more difficult to separate the cells from each other in the gelatin than in the liquid.Advertisement
To effect this some of the nutrient gelatin containing yeast cells is placed on the under-surface of the cover-glass of the moist chamber.
Those cells are accurately marked, the position of which is such that the colonies, to which they give rise, can grow to their full size without coming into contact with other colonies.
Hansen showed that the microscopic appearance of film cells of the same species of Saccharomycetes varies according to the temperature of growth; the limiting temperatures of film formation, as well as the time of its appearance for the different species, also vary.
This observation led him to further work, and he succeeded in showing that in vascular organs the presence of cells in inflammatory exudates is not the result of exudation but of multiplication of pre-existing cells.
The spermatozoa differ from those of other animals in having the form of cells which sometimes perform amoeboid movements.Advertisement
The Daniell type consists of a teak trough divided into five cells by slate partitions coated with marine glue.
The force of the set of accumu- Accumu- lator cells provided is such as to give sufficient power lators.
From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or mothercells of the nematocysts.
The genital cells are simple wandering cells (archaeocytes), at first.
According to Wulfert  the primitive germ-cells of Gonothyraea can be distinguished soon after the fixation of the planula, appearing amongst the interstitial cells of the ectoderm.Advertisement
The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue is greatly developed, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial bodyepithelium.
The sensory cells are slender epithelial cells, often with a cilium or stiff protoplasmic process, and should perhaps be regarded as the only ectoderm-cells which retain the primitive ciliation of the larval ectoderm, otherwise lost in all Hydrozoa.
The sense-cells form, in the first place, a diffuse system of scattered sensory cells, as in the polyp, developed chiefly on the manubrium, the tentacles and the margin of the umbrella, where they form a sensory ciliated epithelium covering the nerve-centres; in the second place, the sense-cells are concentrated to form definite sense-organs, situated always at the margin of the umbrella, hence often termed " marginal bodies."
The statocysts present in general the structure of either a knob or a closed vesicle, composed of (I) indifferent supporting epithelium; (2) sensory, so-called auditory epithelium of slender cells, each.
The uppermost is a purely muscular cell from the sub-umbrella; the two lower are epidermo-muscular cells from the base of a tentacle; the upstanding nucleated portion forms part of the epidermal mosaic on the free surface of the body.Advertisement
Other sensory cells with long cilia cover a sort of cushion (n.c.) at the base of the club; the club may be long and the cushion small, or the...
The process carrying the otolith outer side of a or concretion hk, formed by endoderm cells, is tentacle, two enclosed by an upgrowth forming the " vesicle," nerves run round which is not yet quite closed in at the top. the base of the (After Hertwig.) tentacle to it.
We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu-, latory endoderm, in the radial and ring canals; (3) supporting endoderm in the axes of the tentacles and in the endodermlamella; the latter is primitively a double layer of cells, produced by concrescence OC-- = w.?"
The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.
As described for the polyp, they are wandering cells capable of extensive migrations before reaching the particular spot at which they ripen.Advertisement
If the germ-cells are undifferentiated, the offspring may arise from many cells or from a single cell; the first type is (4) germinal budding, the second is (5) sporogony.
By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily.
The difficulty is solved by the provision of a complete system of minute intercellular spaces which form a continuous series of delicate canals between the cells, extending throughout the whole substance of the plant.
This by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes.
Thallophyta are the most lowly organized plants and include a great variety of forms, the vegetative portion of which consists of a single cell or a number of cells forming a more or less branched thallus.
The male gametophyte is represented by one or few cells and, except in a few primitive forms where the male cell still retains the motile character as in the Pteridophyta, is carried passively to the macrospore in a development of the pollen grain, the pollen tube.
The male gametophyte is sometimes represented by a transitory prothallial cell;, the two male cells are carried passively down into the ovary and into the mouth of the ovule by means of the pollen-tube.
The female gametophyte is extremely reduced; there is a sexual apparatus of naked cells, one of which is the egg-cell which, after fusion with a male cell, divides to form a large siispensorial cell and a terminal embryo.
But all cells which are permanent tissue-elements of the plantbody possess, in addition, a more or less rigid limiting membrane or cell-wall, consisting primarily of cellulose or some allied substance.
F, Section through the surface tissue of the Brown Alga Cutleria multifida, showing the surface layer of assimilating cells densely packed with phaeoplasts.
The layers below have progressively fewer of these, the central cells being quite colorless.
G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed).
In all but the very simplest forms the plant-body is built up of a number of these cells, associated in more or less definite ways.
Such a system is called a tissuesystem, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied.
The word is derived from the general resemblance of the texture of plant substance to that of a textile fabric, and dates from a period when the fundamental constitution of plant substance from individual cells was not yet discovered.
The term parenchyma is applied to tissues whose cells are isodiametric or cylin.drical in shape, prosenchyma tissues consisting of long narrow cells, with pointed ends.
Among the Green Algae the differentiation of cells is comparatively slight.
Many forms, even when multicellular, have all their cells identical in structure and function, and are often spoken of as physiologically unicellular.
The cells Cell and are commonly joined end to end in simple or branched Tissue filaments.
The cells of the axis are commonly stouter and have much less chlorophyll than those of the appendages (Draparnaldia).
The thallus in all cases consists of a branched filament of cells placed end to end, as in many of the Green Algae.
This may have a radial stem-like organization, a central cell-thread giving off from every side a number of short sometimes unicellular branches, which together form a cortex round the central thread, the whole structure having a cylindrical form which only branches when one of the short cell-branches from the central thread grows out beyond the general surface and forms in its turn a new central thread, from whose cells arise new short branches.
In all cases, while the internal threads which bear the cortical branches consist of elongated cells with few chromatophores, and no doubt serve mainly for conduction of food substances, the superficial cells of the branches themselves are packed with chromatophores and form the chief assimilating tissue of the plant.
In the bulky forms colorless branches frequently grow out from some of the cortical cells, and, pushing among the already-formed threads in a longitudinal direction, serve to strengthen the thallus by weaving its original threads together.
The cells belonging to any given thread may be recognized at an early stage of growth, because each cell is connected with its neighbors belonging to the same thread by two depressions or pits, one at each end.
The common wall separating the pits of the two adjoining cells is pierced by strands of protoplasm.
Other pits, connecting cells not belonging to the same branch, are, however, formed at a later stage.
In the Fucaceae, on the other hand, there is a single prismatic apical cell situated at the bottom of a groove at the growing apex of the thallus, which cuts off cells from its sides to add to the peripheral, and from its base to add to the central permanent cells.
The latter are often swollen at the ends, so that the cross-wall separating two successive cells has a larger surface than if the cells were of uniform width along their entire length.
Cells of this type are often called trumpet-hyphae (though they have no connection with the hyphae of Fungi), and in some genera of Laminariaceae those at the periphery of the medulla simulate the sieve-tubes of the higher plants in a striking degree, even (like these latter) developing the peculiar substance callose on or in the perforated cross-walls or sieve-plates.
In addition to the cell types described, it is a very common occurrence in these bulky forms for rhizoid-like branches of the cells to grow out, mostly from the cells at the periphery of the medulla, and grow down between the cells, strengthening the whole tissue, as in the Rhodophyceae.
This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed to very severe fluctuations in external conditions.
These cavities are completely roofed by a layer of cells; in the centre of the roof is a pore surrounded by a ring of special cells.
The frondose (thalloid) Jungermanniales show no such differentiation of an assimilating tissue, though the upper cells of the thallus usually have more chlorophyll than the rest.
In three generaBlyttia, Symphyogyna and Hymenophytum there are one or more strands or bundles consisting of long thickwalled fibre-like (prosenchymatous) cells, pointed at the ends and running longitudinally through the thick midrib.
These cells are not living in the adult state, though they sometimes contain the disorganized remains of protoplasm.
Such differentiated water-conducting cells we call hydroids, the tissue they form hydrom.
The latter are plates of green tissue one cell thick, while the stem consists of uniform more or less elongated cylindrical cells.
In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex.
The whole of the cortex, stereom and parenchyma alike, is commonly living, and its cells often contain starch.
The leaves of most mosses are flat plates, each consisting of a single layer of square or oblong assimilating (chlorophyllous) cells.
In many cases the cells bordering the leaf are produced into teeth, and very frequently they are thick-walled so as to form a supporting rim.
The centre of the leaf is often occupied by a midrib consisting of several layers of cells.
Associated with the leptoids are similar cells without swollen ends and with thicker cross-walls.
Besides the hydrom and leptom, and situated between them, there is a tissue which perhaps serves to conduct soluble carbohydrates, and whose cells are ordinarily full of starch.
The leaf consists of a central midrib, several cells thick, and two wings, one cell thick.
In its centre is a band-shaped bundle consisting of rows of leptom, hydrom and amylon cells.
It is surrounded by (I) a thin-walled, smaller-celled hydrom mantle; (2) an amylom sheath; (3) a leptom mantle, interrupted here and there by starch cells.
As the aerial stem is traced down into the underground rhizome portion, these three mantles die out almost entirelythe central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; at the same time this central hydromstereom strand becomes three-lobed, with deep furrows between the lobes in which the few remaining leptoids run, separated from the central mass by a few starchy cells, the remains of the amylom sheath.
At the periphery of the lobes are some comparatively thin-walled living cells mixed with a few thin-walled hydroids, the remains of the thin-walled hydrom mantle of the aerial stem.
Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plan.t; these are interrupted by strands ofnarrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome.
In the more highly developed series, the mosses, this last division of labor takes the form of the differentiation of special assimilative organs, the leaves, commonly with a midrib containing elongated cells for the ready removal of the products of assimilation; and in the typical forms with a localized absorptive region, a well-developed hydrom in the axis of the plant, as well as similar hydrom strands in the leaf-midribs, are constantly met with.
The gametophyte, which bears the sexual organs, is either a free-living thallus corresponding in degree of differentiation with the lower liverworts, or it is a mass of cells which always remains enclosed in a spore and is parasitic upon the sporophyte.
From the primitive uniform Systems. mass of undifferentiated assimilating cells, which we may conceive of as the starting-point of differentiation, though such an undifferentiated body is only actually realized in the thallus of the lower Algae, there is, (1) on the one hand, a specialization of a surface layer regulating the immediate relations of the plant with its surroundings.
In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of a single layer of cells has typically thickened and cuticularized outer walls.
The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage.
The water stored in such a time supplies the immediate need of the transpiring cells and prevents the injury which would result from their excessive depletion.
Other hairs consist of a chain of cells; others, again, are branched in various ways; while yet others have the form of a flat plate of cells placed parallel to the leaf surface and inserted on a stalk.
The cells of hairs may have living contents or they may simply contain air.
In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connection with the vascular system.
The cells concerned, like all secreting organs, have abundant protoplasm with large nuclei, and sometimes, in addition, part of the cell-wall is modified as a filter.
In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem.
The epithem is frequently surrounded by a sheath of cuticularized cells.
The only pathways for the gases which thus pass between the cells of the mesophyll and the outside air are the stomata.
The main assimilating tissue, on the other hand, is under the upper epidermis, where it is well illuminated, and consists of oblong cells densely packed with chloroplasts and with their long axes perpendicular to the surface (palisade tissue).
The intercellular spaces are here very narrow channels between the palisade cells.
This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through whicF air can pass to the deeper lying tissues.
The cells of these sheaths are often distinguished from the rest of the mesophyll by containing little or no chlorophyll.
These bundle sheaths are important in the conduction of carbohydrates away from the assimilating cells to other parts of the plant.
When, in place of a number of such cells called tracheids, we have a continuous tube with the same kind of wall thickening, but composed of a number of cells whose cross walls have disappeared, the resulting structure is called a vessel.
The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant.
The structure formed by a number of such cells placed end to end is called a sieve-tube (obviously comparable with a xylem-vessel), and the end-wall or area of endwall occupied by a group of perforations, a sseve-plate.
The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells.
The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses).
The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighboring living parenchymatous cells of a growing organ.
The external conjunctive is usually a living comparatively small-celled tissue, whose cells are consider ably elongated in the direction of the stem-axis and frequently contain abundant starch.
As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.
In some cases (Allium, Convolvulaceae, &c.) rows of cells with latex-like contents occur, but the walls separating the individual cells do not break down.
The hypocotyl usually elongates, by its cells increasing very greatly in the longitudinal direction both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves.
In other cases, again, a group of two or four prismatIl cells takes the place of the apical cell.
Segments are then cut if from the outer sides of these initial cells.
Throughout the Angiosperms the epidermis of the shoot originates from separate initials, which never divide tangentially, so that the young shoot is covered by a single layer of dividing cells, the dermatogen.
This however, may be due to irregularity of division and displacement of the cells by irregular tensions destroying the obvious layerec arrangement.
The young tissue of the stelar cylinder, in the case of the modified siphonostele characteristic of the dicotyledonous stem, differs from the adjoining pith and cortex in its narrow elongated cells, a difference produced by the stopping of transverse and the increased frequency of longitudinal divisions.
The protoxylem and protophloem are developed a few cells from the inner and outer margins respectively of the desmogen strand, the desmogenic tissue left over giving rise to the segments of endocycle and pericycle capping the bundle.
Sometimes development stops altogether, and a layer of undifferentiated parenchyma (the mesodesm) is left between them; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side.
The root hairs grow out from the cells of the piliferous layer immediately behind the elongating tegion.
The branches of the stem arise by multiplication of the cells 01 the epidermis and cortex at a given spot, giving rise to a protuber ance, at the end of which an apical meristem is established.
This is known as exogenous branch-formation In the root, on the other hand, the origin of branches is endogenous The cells of the pericycle, usually opposite a protoxylem strand divide tangentially and give rise to a new growing-point.
New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and to the phloem.
A tissue mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to(I) a tracheid; (2) a segment of a vessel; (3) a xylem-fibre; or (4) a vertical file of xylem-parenchyma cells.
When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one plane through the xylem and phloem.
It is essentially a living tissue, and serves to place all the living cells of the secondary vascular tissues in communication.
It conducts plastic substances inwards from the cortex, and its cells are frequently full of starch, which they store in winter.
They are accompanied by intercellular channels serving for the conduction of oxygen to, and carbon dioxide from, the living cells in the interior of the wood, which would otherwise be cut off from the means of respiration.
The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether.
The fibres belong to the same n,orpholcgical category as the parenchyma, various transitions being found between them; thus there may be thin-walled cells of the shape of fibres, or ordinary fibres may be divided into a number of superposed cells.
These intermediate cells, like the ordinary parenchyma, frequently store starch, and the fibres themselves, though usually dead, sometimes retain their protoplasm, and in that case may also be used for starch accumulations.
The stereom is furnished either by cortical cells or by the tracheal elements, in a few cases by fibres which arc probably homologous with sievetubes.
Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube, in other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cellrows of the medullary rays, the middle rows consisting of ordinary starchy cells.
In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type.
The living elements die, and the walls of all the cells often become hardened, owing to the deposit in them of special substances.
In the latter event the cells of the primary rays are either merely stretched radially, or they divide to keep pace with the growth of the bundles.
This consists typically of close-fitting layers of cells with completely suberized walls, intended to replace the epidermis as the external protective layer of the plant when the latter, incapable as it is of further growth after its original formation, is broken and cast off by the increase in thickness of the stem through the activity of the cambium.
The former often has its cells lignified, and may consist of alternate layers of hard and soft cells.
This may take various forms and may cover the whole of the organ or be localized in special regions; but its cells are always living and are separated by very large intercellular spaces containing air.
If we pass a little higher up the scale ot life we meet with forms consisting of two or more cells, each of which contains a similar minute mass of living substance, A study of them shows that each is practically independent of the others; in fact, the connection between them is so slight that they can separate and each becofne free without the slightest disadvantage to another.
Very soon the single cell gives rise to a chain of cells, and this in.
Fuchs and its allies, which form conspicuous members of the larger Algae, have their external cells much smaller, more closely put together, and generally much denser than the rest of their tissue.
The naked cells which have been alluded to live in water, and call therefore for no differentiation in connection with this necessity; but those which are surrounded by a cell-wall always develop within themselves a vacuole or cavity which occupies the greater part of their interior, and the hydrostatic pressure of whose contents keeps tha protoplasm in contact with the membrane, setting up a condition of turgidity.
Certain cells of the exterior are set apart for absorption of water from the soil, this being the source from which supplies are derived.
Other collections of cells are in many cases set apart for giving rigidity and strength to the mass of the plant.
Another kind of differentiation in such a cell-mass as we are dealing with is the setting apart of particular groups of cells for various metabolic purposes.
We have the formation of numerous mechanisms which have arisen in connection with the question of food supply, which may not only involve particular cells, but also lead to differentiation in the protoplasm of those cells, as in the development of the chloroplastids of the leaves and other green parts.
These vary considerably in completeness with its age; in its younger parts the outer cells wall undergoes the change known as cuticularization, the material being changed both in chemical composition and in physical properties.
There is no need for cuticularization here, as the external dangerous influences do not reach the interior, and the processes of absorption which Boussingault attributed to the external cuticularized cells can take place freely through the, delicate cell-walls of the interior, saturated as these are with water.
They are without stomata on their submerged portions, and the entry of gases can only take place by diffusion from the water through their external cells, which are not cuticularized.
The latter ultimately reaches the external air by diffusion through the stomata, whose dimensions vary in proportion as the amount of water in the epidermal cells becomes greater or less.
The entry of gases into, and exit from, the cells, as well as the actual exhalation of watery vapour from the latter, take place in the intercellular space system of which the stomata are the outlets.
The opening and closing of the stomata is the result of variation in the turgidity 01 their guard cells, which is immediately affected by the condition of turgidity of the cells of the epidermis contiguous to them.
The amount of watery vapour in the air passing through a stoma has no effect upon it, as the surfaces of the guard cells abutting on the air chamber are strongly cuticularized, and therefore impermeable.
The only way in which their turgidity is modified is by the entry of water into them from the contiguous cells of the general epidermis and its subsequent withdrawai through the same channel.
There is a distinct advantage in the regulation of this escape, and the mechanism is directly connected with the greater or smaller quantity of water in the plant, and especially in its ep-idermal cells.
Though this at first met with some acceptance, Strasburger showed that the action goes on in great lengths of stem the cells of which have been killed by poison or by the action of heat.
When the tuber of a potato begins to germinate the shoots which it puts out derive their food from the accumulated store of nutritive material which has been laid up in the cells of the tuber.
If we examine the seat of active growth in a young root or twig, we find that the cells in which the organic substance, the protoplasm, of the plant is being formed and increased, are not supplied with carbon dioxide and mineral matter, but with such elaborated material as sugar and proteid substances, or others closely allied to them.
These bodies, known technically as chioroplaIts, are found embedded in the protoplasm of the cells of the mesophyll of foliage leaves, of certain of the cells of some of the leaves of the flower, and of the cortex of the young twigs and petioles.
Usually they are absent from the cells of the epidermis, though in some of the lower plants they are met with there also.
These plastids are especially charged with the duty of manufacturing carbohydrates from the carbon dioxide which the air contains, and which is absorbed from it after it has entered the intercellular passages and has so reached the cells containing the plastids.
It may be, however, that there is no special mechanism, but that this power is a particular differentiation of a physiological kind, existing in all vegetable protoplasm, or in that of certain cells.
The idea of an identity of protoplasm does not involve a denial of special powers developed in it in different situations, and the possession of such a power by the vegetable cell is not more striking than the location of the powers of co-ordination and thought in the protoplasm of cells of the human brain.
They seldom penetrate the living cells, though they do so in a few cases.
Some make their way through the cells of the outer part of the cortex towards the root-tip, and form a mycelium or feltwork of hyphae, which generally occupies two or three layers of cells.
From this branches pass into the middle region of the cortex and ramify through the interior half of its cells.
The food so absorbed passes to the outer cortical mycellum, and from this tc the inner hyphae, which appear to be the organs of the interchangi of substance, for they are attracted to the neighborhood of thi nuclei of the cells, which they enter, and iii which they form agglom erations of interwoven filaments.
The swellings have been found to be due to a curious hypertrophy of the tissue of the part, the cells being filled with an immense number of minute bacterium-like organisms of V, X or Y shape.
When this stage is reached the invading tubes and their ramifications frequently disappear, leaving the cells full of the bacterioids, as they have been called.
Their cells during the period of incubation of the symbiotic organism are abundantly supplied with starch.
The cells in which the fungoid organism is vigorously flourishing are exceedingly active, showing large size, brilliant nuclei, protoplasm and vacuole, all of which give signs of iptense metabolic activity.
This property of living substance can be proved in the case of the cells of the higher plants, but it is especially prominent in many of the more lowly organisms, such as the Bacteria.
There is some evidence pointing to the existence of this power in the cells of the higher plants.
The metabolic changes in the cells, however, concern other decompositions side by side with those which involve the building up of protoplasm from the products of which it feeds.
These include cell walls and the various stored products found in growing cells.
In the lowliest plants growth may be co-extensive with the plantbody; in all plants of any considerable size, however, it is localized in particular regions, and in them it is associated with the formation of new protoplasts or cells.
In other words, as these growing regions consist of cells, the growth of the entire organ or plant will depend upon the behaviour of the cells or protoplasts of which the merismatic tissues are composed.
This is evident from the consideration that the growth of the cells is attended by the growth in surface of the cell wall, and as the latter is a secretion from the protoplasm, such a decomposition cannot readily take place unless oxygen is admitted to it.
Just behind its apex the cells are found to be all in process of active division.
Growth is small, and consists mainly in an increase of the quantity of protoplasm, for the cells divide again as soon as they have reached a certain size.
Here it is that the actual extension in length of the root takes place, and the cells reach the maximum point of the grand period.
The turgidity in the cells of a growing member is not uniform, but shows a fairly rhythmical variation in its different parts.
The differentiation of the plants substance so indicated is, however, physiological only; there is no histological difference between the cells of these regions that can be associated with the several properties they possess.
The root is continually growing and so the sensitive part is continually changing its composition, cells being formed, growing and becoming permanent tissue.
The cells of the tip at any given moment may be sensitive, but in a few days the power of receiving the stimulus has passed to other and younger cells which then constitute the tip. The power of appreciating the environment is therefore to be associated with the protoplasm only at a particular stage of its development and is transitory in its character.
The sensitive cells must clearly be influenced in some way by weightnot the weight of external organs but of some weight within them.
This may possibly be the cell sap in their interior, which must exercise a slightly different hydrostatic pressure on the basal and, the lateral walls of the cells.
Such small granules have been observed in the sensitive cells, and there is an evident correlation between these and the power of receiving the geotropic stimulus.
It has been shown that if the organ containing them is shaken for some time, so that the contact between them and the protoplasm of the cells is emphasized, the stimulus becomes more efficient in producing movement.
The power of response is seen most easily in the case of young growing organs, and the parts which show the motor mechanism are mainly the young growing cells.
The response to the stimulus takes the form of increasing the permeability of particular cells of the growing structures, and so modifying the degree of the turgidity that is the precursor of growth in them.
In the erect position of the leaf the lower side has its cells extremely turgid, and the pulvinus thus forms a cushion, holding up the petiole.
On stimulation these cells part with their water, the lower side of the organ becomes flaccid and the weight of the leaf causes it to fall.
More slowly, but yet in the same way, we may note the change in turgidity of certain cells of the Droscra tentacles, as they close over the imprisoned insect.
The cut cells die, and oxidized products are concerned in the change of color, the brown juices exuding and soaking into the cell-walls.
The next change observable after some hours is that the untouched cells below the cut grow larger, push tip the dead surface, and divide by walls tangential to it, with the formation of tabloid cork-cells.
The layer of cork thus formed cuts out the dead debris and serves to, protect the uninjured cells below.
Irritation and hypertrophy of cells are common signs of the presence of parasites, as ovinced by the numerous malformations, galls, witches-brooms, &c., on diseased plants.
Botrytis, Ergot, &c. Now it is clear that if an organism gains access to all parts of a plant, and stimulates all or most of its cells to hypertrophy, we may have the latter behaving abnormallyi.e.
Phytophthora in potatoes., If, on the other hand, the irritating agent is local in its action, causing only a few cells to react, we have the various pimples, excrescences, outgrowths, &c., exhibited in such cases as Ustilago Maydis on the maize, various galls, witchesbrooms, &c.
It must not be overlooked that the living cells of the plant react upon the parasite as well as to all external agencies, and the nature of disease becomes intelligible only if we bear in mind that it consists in such altered metabolismdeflected physiologyas is here implied.
The reaction of the cells may be in two directions, moreover.
For instance, suppose the effect of a falling temperature is to so modify the metabolism of the cells that they fill up more and more with watery sap; as the freezing-point is reached this may result in destructive changes, and death from cold may result.
If, on the contrary, the gradual cooling is met by a corresponding depletion of the cells of water, even intense cold may be sustained without injury.
The inability to enter the cells may be due to the lack of chemotactic bodies, to incapacity to form cellulose-dissolving enzymes, to the existence in the hostcells of antagonistic bodies which neutralize or destroy the acids, enzymes or poisons formed by the hyphae, or even to the formation and excretion of bodies which poison the Fungus.
But even when inside it does not follow that the Fungus can kill the cell, and many cases are known where the Fungus can break throtigh the cells first lines of defence (cell-wall and protoplasmic lining); but the struggle goes on at close quarters, and various degrees of hypertrophy, accumulation of plastic bodies or secretions, discolorations, &c.,, indicate the suffering of the still living cell.
Among the simplest examples of the former are the hairs which follow the irritation of the cells by mites.
Cecidia or galls arise by the hypertrophy of the subepidermal cells of a leaf, cortex, &c., which has been pierced by theovipositor of an insect, and in which the egg is deposited.
But the occluding callus is a mass of delicate succulent cells, and offers a dainty morsel to certain insects e.g.
Exudations and Rotting.The outward symptoms of many diseases consist in excessive discharges of moisture, often accompanied by bursting of over-turgid cells, and eventually by putrefactive changes.
Chloroplastids are frequently present in the epidermal cells, as in some shade plants.
Forms of stone cells or stereids occur in some of the more suffruticose halophytes, as in Arthrocneniumglaucum.
The epidermal cells may contain chlorophyll.
It is sufficient to note here that cells were first of all discovered in various vegetable tissues by Robert Hooke in 1665 (Micrographia); Malpighi and Grew (1674-1682) gave the first clear indications of the importance of cells in the building up of plant tissues, but it was not until the beginning of the 19th century that any insight into the real nature of the cell and its functions was obtained.
The nucleus was definitely recognized in the plant cell by Robert Brown in 1831, but its presence had been previously indicated by various observers and it had been seen by Fontana in some animal cells as early as 1781.
He showed that all the organs of plants are built up of cells, that the plant embryo originates from a single cell, and that the physiological activities of the plant are dependent upon the individual activities of these vital units.
It is true that in the unicellular plants all the vital activities are performed by a single cell, but in the multicellular plants there is a more or less highly developed differentiation of physiological activity giving rise to different tissues or groups of cells, each with a special function.
Young cells ar full of cytoplasm, old cells generally contain a large vacuole or vacuoles, containing cell-sap, and with only a thin, almost invisible layer of cytoplasm on their walls.
This structure, which is visible both in living cells and in.
Protoplasmic Movements.In the cells of many plants the cytoplasm frequently exhibits movements of circulation or rotation.
The cells of the staminal hairs of Tradescantia air ginica contain a large sap-cavity across which run, in.
In some cases both the nucleus and the chromatophores may be carried along in the rotating stream, but in others, such as T.Titeila, the chloroplasts may remain motionless iii a non-motile layer of the cytoplasm in direct contact with the cell wall.i Desmids, Diatoms and Oscillaria show creeping movements probably due to the secretion of slime by the cells; the swarmspores and plasmodium of the Myxomycetes exhibit amoehoid movements; and the motile spores of Fungi and Algae, the spermatozoids of mosses, ferns, &c., move by means of delicate prolongations, cilia or flagella cf the protoplast.
In young cells the chromatophores are small, colorless, highly refractive bodies, principally located around the nucleus.
Substances contained in the Protoplastn.Starch may be found in the chlorophyll bodies in the form of minute granules as the first visible product of the assimilation of carbon dioxide, and it occurs in large quantities as a reserve food material in the cells of various parts of plants.
HaberIandt has shown that in plant cells, when any new formation of membrane is to take place in a given spot, the nucleus is found in its immediate vicinity; and Klebs found that only that portion of the protoplasm of a cell which contains the nucleus is capable of forming a cell-wall; whilst Townsend has further shown that if the non-nucleated mass is connected by strands of protoplasm to the nucleated mass, either of the same cell or of a neighboring cell, it retains the power of forming a cell-membrane.
Centrosome.The centrosome is a minute homogeneous granule found in the cytoplasm of some cells in the neighborhood of the nucleus.
In plant cells its presence has been demonstrated in the Thallophytes and Bryophytes.
Nuclear Division.The formation of new cells is, in the case of tminucleate cells, preceded by or accompanied by the division of the nucleus.
In multinucleate cells the division of the nucleus is independent of the division of the cell.
Tradescantia; and in various other cells which have lost their power of division.
It has been shown that, in cells of Spiro gyra placed under special conditions, amitotic division can be induced, and that normal mitosis is resumed when they are placed again under normal conditions.
It is clear, however, that an equal quantitative division and distribution of the chromatin to the daughter cells is brought about; and if, as has been suggested, the chromatin consists of minute particles or units which are the carriers of the hereditary characteristics, the nuclear division also probably results in the equal division and distribution of one half of each of these units to each daughter cell.
In the vascular cryptogams and phanerogams it takes place in the spore mother cells and the reduced number is found in all the cells of the gametophyte, the full number in those of the sporophyte.
Cell Division.With the exception of a few plants among the Thallophytes, which consist of a single multinucleate cell, Caulerpa, Vaucheria, &c., the division of the nucleus is followed by the division of the cell either at once, in uninucleate cells, or after a certain number of nuclear divisions, in multinucleate cells.
In a few cases both among the higher and the lower plants, of which the formation of spores in the ascus is a typical example, new cells are formed by the aggregation of portions of the cytoplasm around the nuclei which become delimited from the rest of the cell iontents by a membrane.
It is thickened more in some places than in others, and thus are formed the spiral, annular and other markings, as well as the pits which occur on various cells and vessels.
Besides the internal or centripetal growth, some cell-walls are thickened on the outside, such as pollen grains, oospores of Fungi, cells of Peridineae, &c. This centrifugal growth must apparently take place by the activity of protoplasm external to the cell.
Cuticularized or suberized cell-walls occur especially in those cells which perform a protective function.
Both cuticularized and suberized membranes are insoluble in cuprammonia, and are colored yellow or brown in a soltition of chlor-iodide of zinc. It is probable that the corky or suberized cells do not contain any cellulose (Gilson, Wisselingh); whilst cuticularized cells are only modified in their outer layers, cellulose inner layers being still recognizable.
Fertilization.The formation of the zygote or egg-cell takes place usually by the fusion of the contents of two cells, and always includes, as -
In plants with multinucleate cells, such as Albugo, Peronospora and Vaucheria, it is usually a uninucleate cell differentiated by separation of the nuclei from a multinucleate cell, but in Albugo bliti it is multinucleate, and in Sphaero plea it may contain more than one nucleus.
The strongest direct evidence seems to be that the nuclear substances are the only parts of the cells which are always equivalent in quantity, and that in the higher plants and animals the male organ or spermatozoid is composed almost entirely of the nucleus, and that the male nucleus is carried into the female cell without a particle of cytoplasm.i Since, however, the nucleus of the female cell is always accompanied by a larger or smaller quantity of cytoplasm, and that in a large majority of the power plants and animals the male cell also contains cytoplasm, it cannot yet be definitely stated that the cytoplasm does not play some part in the process.
The cytological evidence for this appears to be made stronger for animal than for plant cells.
Organs which respond to the mechanical stimulus of contact are found to possess special contrivances in certain of their cells(I) sensitive spots, consisting of places here and there on the epidermal cells where the wall is thin and in close contact with protoplasmic projections.
Notwithstanding the fact, however, that these cells are capable of acting as very efficient lenses the explanation given by Haberlandt has not been widely accepted and evidence both morphological and physiological has been brought forward against it.
Tissues.The component parts of the tissues of which plants are composed may consist of but slightly modified cells with copious protoplasmic contents, or of cells which have been modified in various ways to perform their several functions.
The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them.
Tubes formed by the elongation of single cells are found in bast fibres, tracheides, and especially in laticiferous cells.
Laticiferous vessels arise by the coalescence of originally distinct cells.
The cells not only fuse together in longitudinal and transverse rows, but put out transverse projections, which fuse with others of a similar nature, and thus form an anastomosing network of tubes which extends to all parts of the plant.
The rows of cells from which the laticiferous vessels are formed can be distinguished in many cases in the young embryo while still in the dry seed (Scott), but the latex vessels in process of formation are more easily seen when germination has begun.
The opening, which is at first very small, increases in size, and before the cross-wall has entirely disappeared the contents of the two cells become continuous (Scott).
Sieve Tubes.The sieve tubes consist of partially fused rows of cells, the transverse cr lateral walls being perforated by minute openings, through which the contents of the cells are connected with each other, and which after a certain time become closed by,the formation of callus on the sieve plates.
Companion cells are not found in the Pteridophyta and Gymnosperms. In the latter their place is taken by certain cells of the medullary rays and bast parenchyma.
The companion cells are cut off from the same cells as those which unite to form the sieve tube.
As the sieve plate grows these non-cellulose regions swell and gradually become converted into the same kind of mucous substance as that contained in the tube; the two cells are thus placed in open communication.
The presence of these threads between all the cells of tfie plant shows that the plant body must be regarded as a connected whole; the threads themselves probably play an important part in the growth of the cell-wall, the conduction of food and water, the process of secretion and the transmission of impulses.
The cells of the Inquisition were, as a rule, large, airy, clean and with good windows admitting the sun.
The kiln consists of two (or more) connected cells which are both charged with the ore.
On the 20th of October 1349 Clement published a bull commanding the bishops and inquisitors to stamp out the growing heresy, and in pursuance of the pope's orders numbers of the sectaries perished at the stake or in the cells of the inquisitors and the episcopal justices.
Dubois (1886), who considers that the luminosity is due to the influence of an enzyme in the cells of the organ upon a special substance in the blood.
The grubs, when hatched, start galleries nearly at right angles to this, and when fully grown form oval cells in which they pupate; from these the young beetles emerge by making circular holes directly outward through the bark.
Following up this line of investigation, Major Ronald Ross in 1895 found that if a mosquito sucked blood containing the parasites they soon began to throw out flagellae, which broke away and became free; and in 1897 he discovered peculiar pigmented cells, which afterwards turned out to be the parasites of aestivo-autumnal malaria in an early stage of development, within the stomachwall of mosquitoes which had been fed on malarial blood.
He further found that only mosquitoes of the genus Anopheles had these cells, and that they did not get them when fed on healthy blood.
Then, turning his attention to the malaria of birds, he worked out the life-history of these cells within the body of the mosquito.
Moreover, the pollen, instead of consisting of separate cells or grains, consists of cells aggregated into "pollen-masses," the number varying in different genera, but very generally two, four, or eight, and in many of the genera provided at the base with a strap-shaped stalk or "caudicle" ending in a flattish gland or "viscid disk" like a boy's sucker.
They derive this moisture from the air by means of aerial roots, developed from the stem and bearing an outer spongy structure, or velamen, consisting of empty cells kept open by spiral thickenings in the wall; this sponge-like tissue absorbs dew and rain and condenses the moisture of the air and passes it on to the internal tissues.
A single layer of epidermic cells, some of which are glandular, forms the outer layer.
The coelom is lined throughout by cells, which upon the intestine become large and loaded with excretory granules, and are known as chloragogen cells.
Several forms of cells float freely in the fluid of the coelom.
In the latter, the segmentally arranged ganglia are more sharply marked off from the connectives than in other Chaetopods, where nerve cells exist along the whole ventral chain, though more numerous in segmentally disposed swellings.
It has been asserted (and denied) that the cellular rod which is known as the "Heart-body" (Herzkorper), and is to be found in the dorsal vessel of many Oligochaeta and Polychaeta, is formed of cells which are continuous with the chloragogen cells, thus implying the existence of apertures of communication with the coelom.
Vezhdovsky has lately seen reasons for regarding the blood system as originating entirely from the hypoblast by the secretion of fluid, the blood, from particular intestinal cells and the consequent formation of spaces through pressure, which become lined with these cells.
Essentially, a nephridium is a tube, generally very long and much folded upon itself, composed of a string of cells placed end to end in which the continuous lumen is excavated.
Such cells are termed "drain pipe" cells.
Frequently the lumen is branched and may form a complicated anastomosing network in these cells.
The funnel varies greatly in size and number of its component cells.
The epidermis contains numerous groups of sense cells; beneath the epidermis there is rarely (Kynotus) an extensive connective tissue dermis.
In the aquatic genera the epidermis comes to consist entirely of glandular cells, which are, however, arranged in a single layer.
In the earthworms, on the other hand, the epidermis becomes specialized into several layers of cells, all of which are glandular.
It is further noticeable that in Rhynchelmis the covering of vesicular cells which clothes the drainpipe cells of the adult nephridium is cut off from the nephridial cells themselves and is not a peritoneal layer surrounding the nephridium.
These sacs contain the developing sperm cells or eggs, and are with very few exceptions universal in the group. The testes are more commonly thus involved than are the ovaries.
In the middle of the body, where the limits of the somites can be checked by a comparison with the arrangement of the nephridia and the gonads, and where the ganglia are quite distinct and separated by long connectives, each ganglion is seen to consist of six masses of cells enclosed by capsules and to give off three nerves on each side.
The brain con sists not only of a group of six capsules corre sponding to the archi cerebrum of the Oligo chaeta, but of a further mass of cells surrounding S S and existing below the alimentary canal, which can be analysed into five or six more separate ganglia.
The nephridia are like those of the Oligochaeta in general structure; that is to say, they consist of drain-pipe cells which are placed end to end and are perforated by their duct.
The internal funnel varies in the same way as in the Oligochaeta in the number of cells which form it.
In Clepsine (Glossiphonia) there are only three cells, and in Nephelis five to eight cells.
In Hirudo the funnel is not pervious and is composed of a large number of cells.
This coelom is lined by peritoneal cells and is divided into a series of metameres by septa which correspond to the segmentation of the FIG 15.
Moreover, upon the intestine the coelomic cells are modified into chloragogen cells.
In Hirudo and the Gnathobdellidae there is only one system of cavities which consist of four principal longitudinal trunks, of which the two lateral are contractile, which communicate with a network ramifying everywhere, even among the cells of the epidermis.
The network is partly formed out of pigmented cells which are excavated and join to form tubes, the socalled botryoidal tissue, not found among the Rhynchobdellidae at all.
The ovaries are solid bodies, of which the outer layer becomes separated from the plug of cells lying within; thus a cavity is formed which is clearly coelom.
The main distinction is the occurrence in the tissue of the fruit, or beneath the rind, of clusters of cells filled with hard woody deposit in the case of the pear, constituting the "grit," while in the apple no such formation of woody cells takes place.
As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes.
It is surrounded by a ridge of cells which gradually extends over the visceral sac and secretes the shell.
Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified cells placed in sacs (cnidosacs) at the apices of these same papillae, which resemble the " thread-cells " of the Coelentera.
Some form their diblastula by emboly, others by epiboly; and in the later history of the further development of the enclosed cells (archenteron) very marked variations occur in closely-allied forms, due to the influence of a greater or less abundance of food-material mixed with the protoplasm of the egg.
There is not a very large amount of food-material present in the egg of this snail, and accordingly the cells resulting from division are not so unequal as in many other cases.
Then the large cells recommence the process of division and sink into the hollow of the sphere, leaving an elongated groove, the blastopore, on the surface.
The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.
The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the " rectal peduncle " or " pedicle of invagination " of Lankester, although the endoderm cells retain no contact with the middle region of the now closed-up blastopore.
The body-cavity and the muscular, fibrous and vascular tissues are traced partly to two symmetrically disposed " mesoblasts," which bud off from the invaginated arch-enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm.
When the middle and hinder regions of the blastopore are closing in, an equatorial ridge of ciliated cells is formed, converting the embryo into a typical trochosphere.
The mass of the arch-enteron or invaginated endodermal sac has taken on a bilobed form, and its cells are swollen (gs and tge).
A remarkable cord of cells having a position just below the integument occurs on each side of the head.
This paired organ consists of a string of cells which are perforated by a duct opening to the exterior and ending internally in a flame-cell.
Such cannulated cells are characteristic of the nephridia of many worms, and the organs thus formed in the embryo Limnaeus are embryonic nephridia.
These dorsal eyes are very perfect in elaboration, possessing lens, retinal nerve-end cells, retinal pigment and optic nerve.
Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re surfaces of the nerve-end cells nearer the lens instead of with the opposite end.
Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells.
These nervures consist of a series of trunks radiating from the wing-base and usually branching as they approach the wing-margins, the branches being often connected by short transverse nervures, so that the wing-area is marked off into a number of " cells " or areolets.
An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and y a chitinous layer internally, the lastnamed being continuous with the outer cuticle.
The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding bodycavity.
When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion.<