In the case of the great grey kangaroo, for instance, the period of gestation is less than forty days, and the newly-born embryo, which is blind, naked, and unable to use its bud-like limbs, is little more than an inch in length.
(2) To the second class belong Ankylostoma, Strongylus and many species of Ascaris; the embryo on leaving the egg lives free in water or damp earth, and resembles very closely the free-living genus Rhabditis.
The female is viviparous, and the young, which, unlike the parent, are provided with a long tail, live free in water; it was formerly believed from the frequency with which the legs and feet were attacked by this parasite that the embryo entered the skin directly from the water, but it has been shown by Fedschenko, and confirmed by Manson, Leiper and others, that the larva bores its way into the body of a Cyclops and there undergoes further development.
If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa.
If the embryo is set free as a free-swimming, so-called planula-larva, in the blastula, parenchymula, or gastrula stage, then a free actinula stage is not found; if, on the other hand, a free actinula occurs, then there is no free planula stage.
There is no free-swimming planula larva, but the stage corresponding to it is passed over in an enveloping cyst, which is secreted round the embryo by its own ectodermal layer, shortly after the germ-layer formation is complete, i.e.
The parasite effects a lodgment in the host either by invading it as a free-swimming planula, or, apparently, in other cases, as a spore-embryo which is captured and swallowed as food by the host.
Chun and Woltereck, on the other hand, regard the stem as a stolo prolifer arising from the aboral pole, that is to say, from the ex-umbrella, similar to that which grows out from the ex-umbral surface of the embryo of the Narcomedusae and produces buds, a view which is certainly supported by the embryological evidence to be adduced shortly.
In fact, there is a period when, as Aristotle long ago said, the embryo of the highest animal has the form of a mere worm, and, devoid of internal and external organization, is merely an almost structureless lump of polype-substance.
It is not true, for example, that a fish is a reptile arrested in its development, or that a reptile was ever a fish; but it is true that the reptile embryo, at one stage of its development, is an organism which, if it had an independent existence, must be classified among fishes; and all the organs of the reptile pass, in the course of their development, through conditions which are closely analogous to those which are permanent in some fishes.
It is absurd to call the larva of a newt or of a Caecilian a tadpole, nor is the free-swimming embryo of a frog as it leaves the egg a tadpole.
The spores, as in the heterosporous Pteridophyta, are of two kindsmicrospores (pollen grains) borne in microsporangia (pollen sacs) on special leaves (sporophylls) known as stamens, and macrospores (embryo-sac) borne in macrosporangia (ovules) on sporophylls known as carpels.
After fertilization the female cell, now called the oospore, divides and part of it develops into the embryo (new sporophyte), which remains dormant for a time still protected by the ovule which has developed to become the seed.
The embryo consists of an axis bearing two or more cotyledons and ending below in a radicle; it lies in a generally copious food-storing tissue (endosperm) which is the remains of the female prothallus.
Endosperm is formed as the result of the fusion of the second male cell with the so-called definitive nucleus of the embryo-sac (see ANGlOSPERMS).
The embryo consists of an axis bearing one (Monocotyledons) or two (Dicotyledons) cotyledons, which protect the stem bud (plumule) of the future plant, and ending below in a radicle.
The body thus formed ment of is called the embryo, and this develops into the adult Primary plant, not by continued growth of all its parts as in an animal, but by localization of the regions of cell-division and growth, such a localized region being called a growing-point.
Later, the axis branches by the formation of new growing-points, and in this way the complex system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stopf growing and rests, enclosed within the seed.
In cases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upperpart of which often shows stem-structure, in some respects at least, and is called the hypocoty- ledonary stem or hypocotyl, while the lower part is the primary root .~-,
When the young sporophyte first begins its independent lifewhen, that is, it exists in the form of the embryo in the seedits living substance has no power of utilizing the simple inorganic compounds spoken of.
If we go back to the first instance cited, the embryo in the seed and its development during germination, we can ascertain what is necessary for its life by inquiring what are the materials which are deposited in the seed, and which become exhausted by consumption as growth and development proceed.
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.
Both nuclei are elongated vermiform structures, and as they enter the embryo sac present a twisted appearance like a spermatozoid without cilia (fig.
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.
Xxi.; On the Prophases of the Heterotype Mitosis in the Embryo-sac Mother-cell of Lilium, Ann.
It is absent in the Ratitae, which from this feature have received their name, but considerable traces of a cartilaginous keel occur in the embryo of the ostrich, showing undeniably that the absence of a keel in the recent bird is not a primitive, fundamental feature.
The name expresses the most universal character of the class, the importance of which was first noticed by John Ray, namely, the presence of a pair of seed-leaves or cotyledons, in the plantlet or embryo contained in the seed.
The embryo is generally surrounded by a larger or smaller amount of foodstuff (endosperm) which serves to nourish it in its development to form a seedling when the seed germinates; frequently, however, as in pea or bean and their allies, the whole of the nourishment for future use is stored up in the cotyledons themselves, which then become thick and fleshy.
In germination of the seed the root of the embryo (radicle) grows out to get a holdfast for the plant; this is generally followed by the growth of the short stem immediately above the root, the so-called "hypocotyl," which carries up the cotyledons above the ground, where they spread to the light and become the first green leaves of the plant.
The seeds are minute and innumerable; they contain a small rudimentary embryo surrounded by a thin loose membraneous coat, and are scattered by means of hygroscopic hairs on the inside of the valves which by their movements jerk out the seeds.
Vezhdovsky to exist in the embryo of certain forms. The blood in the Chaetopoda consists of a plasma in which float a few corpuscles.
It has been shown (Bourne) that the "perichaetous" condition is probably secondary, inasmuch as in worms which are, when adult, "perichaetous" the setae develop in pairs so that the embryo passes through a stage in which it has four bundles of setae, two to each bundle, the prevalent condition in the group. Rarely there is an irregular disposition of the setae which are not paired, though the total number is eight to a segment (fig.
B, Embryo with ventral flexure of the intestine.
C, Embryo with ventral flexure and exogastric shell.
D, Embryo with lateral torsion and an endogastric shell.
But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shellgland, passing through the earlier condition of a trochosphere to that of the veliger.
Areas are found on the foot of the embryo Pulmonate Limax and on the yolk-sac (distended foot-surface) of the Cephalopod Loligo.
The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore.
The Pulmonata have a straight visceral nerve-loop, usually no operculum even in the embryo, and a multidenticulate radula, the teeth being equi-formal; and they are hermaphrodite.
The foot is always simple, with its flat crawling surface extending from end to end, but in the embryo Limnaea it shows a bilobed character, which leads on to the condition characteristic of Pteropoda.
In Limnaeus the permanent shell is preceded in the embryo by a wellmarked shell-gland or primitive shell-sac (fig.
- Embryo of Limnaeus stagnalis, at a stage when the Trochosphere is developing foot and shell-gland and becoming a Veliger, seen as a transparent object under slight pressure.
The external form of the embryo goes through the same changes as in other Gastropods, and is not, as was held previously to Lankester's observations, exceptional.
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.
Such cannulated cells are characteristic of the nephridia of many worms, and the organs thus formed in the embryo Limnaeus are embryonic nephridia.
- The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band.
And of the muscid flies, an anterior and a posterior endodermThe embryo thus becomes transferred to the dorsal face of the egg, rudiment both derived from the " endoblast " become apparent but at a later stage it undergoes reversion to its original ventral at an early stage, in close association with the stomodaeum and position.
On this view Wheeler, however, compares with the " dorsal organ " the peculiar the entire food-canal in most Hexapoda must be regarded as of extra embryonic membrane or indusium which he has observed ectodermal origin, the " endoblast " represents mesoderm only, between serosa and amnion in the embryo of the grasshopper and the median furrow whence it arises can be no longer compared Xiphidium.
Size compared with the thorax or abdomen, but in the embryo it On the whole it seems likely that the endoderm is represented in forms a much larger portion of the body than it does in the adult.
The other is the segment of the maxillulae (see above, under Jaws), behind the mandibular somite; the presence of this in the embryo of the collembolan Anurida has been lately shown (1900) by J.