The primitive shell-sac or shell-gland is well marked at this stage, and the pharynx is seen as a new ingrowth (the stomodaeum), about to fuse with and open into the primitively invaginated arch-enteron (fig.
The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.
Ing gs, the bilobed arch sh, The primitive shell-sac or enteron or lateral vesicles shell-gland ° of invaginated endoderm, pi, The rectal peduncle or whichwill develop into liver.
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.
The mass of the arch-enteron or invaginated endodermal sac has taken on a bilobed form, and its cells are swollen (gs and tge).
To the right (in the figure) of the rectal peduncle is seen the deeply invaginated shell-gland ss, with a secretion sh protruding from it.
Kowalevsky or a bug is invaginated into the yolk at the head end, the portion of (1871 and 1887) on the embryology of the water-beetle Hydrophilus the blastoderm necessarily pushed in with it forming the amnion.
Stated that in the Muscidae, while the anterior endoderm-rudiment The embryo is invaginated into the yolk, but the surface edges of arises as Kowalevsky had observed, the posterior part of the " midthe blastoderm do not close over, so that a groove or pore puts gut " has its origin as a direct outgrowth from the proctodaeum.
Lecaillon (1898) on various leaf beetles, tend to show that the organ " in the embryos of the lower Arthropoda corresponds with whole of the " mid-gut " arises from the proliferation of cells at the the region invaginated to form the serosa of the hexapod embryo.
Their contraction causes the proboscis to be invaginated into its cavity (fig.
B, A section through the same; a, the invaginated proboscis; b, proboscis sheath; c, beginning of the neck; d, lemniscus.
B, invaginated head of a Cysticercus before the formation of the suckers; X 25.
C, invaginated head of Cysticercus cellulosae, showing the bent neck and receptacle r; X 30.
By a process of infolding, the thicker end is partially invaginated, the middle portion or " hind-body " and the organism may now present a superficial likeness to a cercaria.
B, portion of the bladder (hind-body and tail), showing the invaginated portion (scolex) and traces of the excretory system.
In most other cases the tail is not distinguishable, and the body of the larva is separable only into a scolex invaginated with a bladder (= hind-body and tail).
In accordance with this we can regard the development as an adaptive one and the scolex as invaginated for protective FIG.
(Modified from Horst.) A, Blastula stage (one-cell-layered eaten its way into the in sac), with commencing invaginated endodermal sac, vagination of the wall of the and the cells pushed in with sac at bl, the blastopore.
25), no part of the blastopore persists either as mouth or as anus, but the aperture closes - the pedicle of invagination, or narrow neck of the invaginated arch-enteron, becoming the intestine.
The tion of liquid between the gonads themselves are extremely outer and the invaginated simple arborescent glands which cells.
24, in optical median section, showing the invaginated cells hy which form the arch-enteron, and the mesoblastic cells me which are budded off from the surface of the mass hy, and apply themselves to the inner surface of the epiblastic cell-layer cp. C, The same embryo focused so as to show the mesoblastic cells which immediately underlie the outer cell-layer.
24); within this, at one pole, is seen the mass of invaginated cells (fig.
These invaginated cells are the archenteron; they proliferate and give off branching cells, which apply themselves (fig.
The invaginated layer is the enteric cell-layer or endoderm; the outer cell-layer is the dermic cell-layer or ectoderm.
The shell-gland, as development proceeds, extends from its point of origin as an ectodermic thickening, which may be only slightly concave or may be deeply invaginated and then evaginated.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.