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.
Med., Longitudinally running comparatively colorless central (medullary) branches, which conduct food substances and support the (ass.
Primary, medullary, ray.
These collateral bundles are separated from one another by bands of conjunctive tissues called primary medullary rays, which may be quite narrow or of considerable width.
The pericycle, medullary rays, endocycle and mesoderm all form parts of one tissue system, the external conjunctive, and are only topographically separable.
Such a cell-plate is called a medullary ray.
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 cortical tissues gradually shrink and dry up, turning brown and black in patches or all over, and when at length the cambium and medullary ray tissues dry up the whole twig dies off.
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 nervous system is thus essentially epidermal in position and diffuse in distribution; but an interesting concentration of nerve-cells and fibres has taken place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores.
Sometimes the central canal is wide and uninterrupted between the two neuropores; in other cases it becomes broken up into a large number of small closed medullary cavities, and in others again it is obsolete.
In one family, the Ptychoderidae, the medullary tube of the collar is connected at intermediate points with the epidermis by means of a variable number of unpaired outgrowths from its dorsal wall, generally containing an axial lumen derived from and in continuity with the central canal.
An important feature is the occurrence in some species (Ptychoderidae) of paired longitudinal pleural or lateral folds of the body which are mobile, and can be approximated at their free edges so as to close in the dorsal surface, embracing both the median dorsal nerve-tract and the branchial grooves with the gill-pores, so as to form a temporary peri-branchial and medullary tube, open behind where the folds cease.
Related to this substance are " neuro-keratin," found in the medullary sheath of nerves, and " gorgonin," the matrix of the axial skeleton of the coral Gorgonia Cavolinii.
Ce, Cerebral arteries; sp, supra - spinal or medullary artery; c, caudal artery; 1, lateral anastomotic artery of Limulus.
The ita palm, Mauritia, flexuosa (a fanleaf palm) provides an edible fruit, medullary meal, drink, fibre, roofing and timber, but is less used on the Amazon than it is on the lower Orinoco.
Another highly useful palm is the carnauba or carnahuba (Copernicia cerifera) which supplies fruit, medullary meal, food for cattle, boards and timber, fibre, wax and medicine.
From the great size of the trunk and the even grain of the red cedar-like wood it is a valuable tree to the farmer and carpenter: it splits readily and evenly, and planes and polishes well; cut radially, the medullary plates give the wood a fine satiny lustre; it is strong and durable, but not so elastic as many of the western pines and firs.
The typical heteromerous thallus shows on section a peripheral, thin and therefore transparent, layer, the cortical layer, and centrally a mass of denser tissue the so-called medullary layer, between these two layers is the algal zone or gonidial layer (figs.
The surface of the thallus often exhibits outgrowths in the form of warts, hairs, &c. The medullary layer, which usually forms the main part of the thallus, is distinguished from the cortical layer by its looser consistence and the presence in it of numerous, large, air-containing spaces.
M, Medullary layer.
M, Medullary hyphae of u, Under rind.
M, Medullary layer of the C, Cladonia novae Angliae, B, Portion of a very thin Delise; sterigmata section from the base with spermatia from of the spermogonium.
C, Medullary layer.
The medullary rays extend radially from the centre of the tree to the bark at right angles to the grain of the wood, and serve during life to bind the whole together as well as to convey nourishment from one part of the tree to another.
In oak this develops the beautiful silver grain by cutting longitudinally through the medullary rays.
No medullary rays are visible; the wood is straight in the grain, durable, strong and elastic, easy to work, and is used by the carpenter for internal and external constructional work, and by the joiner for his fittings.
The wood is very hard, tough, with fine regular grain and close texture, the annular rings being distinct and the medullary rays well marked.
The primordium of the neurochord (neural or medullary plate) referred to above becomes closed in from the surface by the overgrowth of surrounding epiblast, and its edges also bend up, meet, and finally fuse to form a tube, the medullary or neural tube.
The timber bears a striking resemblance to that of the oak, which has been mistaken for chestnut; but it may be distinguished by the numerous fine medullary rays.
The pith is encircled by a cylinder of secondary wood, consisting of single or multiple radial rows of tracheids separated by broad medullary rays composed of large parenchymatous cells; the tracheids bear numerous bordered FIG.
The large medullary rays give to the wood a characteristic parenchymatous or lax appearance, which is in marked contrast to the more compact wood of a conifer.
The xylem being centrifugal and the m, Medullary bundles.
Abnormal female of the medullary rays in Flowers.
The secondary phloem contains numerous thick-walled fibres, parenchymatous cells, and large sieve-tubes with plates on the radial walls; swollen parenchymatous cells containing crystals are commonly met with in the cortex, pith and medullary-ray tissues.
The genus Pinus serves as an illustration of wood of a distinct type characterized by the absence of xylemparenchyma, except such as is associated with the numerous resincanals that occur abundantly in the wood, cortex and medullary rays; the medullary rays are composed of parenchyma and of horizontal tracheids with irregular ingrowths from their walls.
The medullary rays usually consist of a single tier of cells, but in the Pinus type of wood broader medullary rays also occur and are traversed by horizontal resin-canals.
An examination of the wood of branches, stems and roots of the same species or individual usually reveals a fairly wide variation in some of the characters, such as the abundance and size of the medullary rays, the size and arrangement of pits, the presence of wood-parenchyma - characters to which undue importance has often been attached in systematic anatomical work.
When tracheids occur in the medullary rays of the xylem these are replaced in the phloem-region by irregular parenchymatous cells known as albuminous cells.
An anatomical peculiarity in the veins of Pinus and several other genera is the continuity of the medullary rays, which extend as continuous plates from one end of the leaf to the other.
It is of interest to note that the leaves of Gnetum, while typically Dicotyledonous in appearance, possess a Gymnospermous character in the continuous and plate-like medullary rays of their vascular bundles.
The coenenchyma is further differentiated into a medullary portion and a cortex.
The medullary portion is densely crowded with spicules of different shape from those in the cortex, and in some forms the spicules are cemented together to form a hard supporting axis.
In some mammals, as deer, the substance of the hair is almost entirely composed of the central medullary or cellular substance, and is consequently very easily broken; in others the horny part prevails almost exclusively, as in the bristles of the wild boar.
His theory assumes the correspondence of mind and body, and is applied pari passu to the formation of ideas from sensations, and of " compound vibratiuncules in the medullary substance " from the original vibrations that arise in the organ of sense.2 The same general view was afterwards developed with much vigour and clearness on the psychical side alone by James Mill in his Analysis of the Human Mind.
Mistakes have often arisen from confusing these medullary casts with those of the stem as a whole.
Radially placed gaps in the tissue (at first erroneously interpreted as medullary rays, but subsequently more aptly compared to the air-spaces of large Algae) contain very sparse hyphae, which here branch more freely than elsewhere.
(I) carbonaceous impressions of the leafy branches, the fructifications and other parts; (2) casts of the stem; these are usually internal, or medullary casts, as described above.
More usually, however, the carbonaceous film is thin, and merely shows the impress of the medullary cast within; (3) petrified specimens of all parts - stem, roots, leaves and fructifications - showing the internal structure, more or less perfectly preserved.
B entirely of radial bands of tracheides interspersed with medullary rays.
Insigne, has scalariform tracheides, and is traversed by regular medullary rays, but in the forms from later ' horizons the tracheides are reticulately pitted, and the rays are for the most part replaced by a network of xylem-parenchyma.
The curious, transversely-ribbed fossils known as Sternbergia or Artisia have proved to be casts of the medullary cavity of Cordaiteae; their true nature was first demonstrated by Williamson in 1850.
18 Greenland, and other Arctic lands and G12 G13 Gla G15 G16 G17 Yale University has noticed in some of the Cycadean stems from the Black hills of Dakota and Wyoming that the wood appears to possess a similar structure, differing in its narrower medullary rays from the wood of modern Cycads.
- In large doses the action of picrotoxin is exerted chiefly on the medullary nerve centres, whereby irregular tonic-clonic convulsions are produced; in minute doses it stops the secretion of sweat.
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.