The blastopore is now more F, Later embryo seen as a transcontracted, d; and cells, me, m, Mouth.
Formation of archenteron and blastopore may, however, be deferred till a later stage (actinula or after).
Bobretzky, the closed blastopore may coincide in position with the mouth in some instances (Nassa, &c.), instead of with the anus.
In this development the blastopore is not elongated; it persists as the anus.
C, Diblastula of an Opisthobranch (Polycera) with elongated blastopore oi.
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 blastopore now closes along the middle part of its course, which coincides z s FIG.
Elongated blastopore of fig.
One end of the blastopore becomes nearly closed, and an ingrowth of ectoderm takes place around it to form the stomodaeum or fore-gut and mouth.
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.
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.
We may now revert briefly to the internal organization at a period when the trochosphere is beginning to show a prominent foot growing out from the area where the mid-region of the elongated blastopore was situated, and having therefore at one end of it the mouth and at the other the anus.
It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.
Round the blastopore hollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm.
They surround a region which is termed the peristome, and which contains in the centre the blastopore, which becomes the adult mouth.
In the anthopolyp the blastopore is carried inwards by an in-pushing of the body-wall of the region of the peristome, so that the adult mouth is an opening leading into a short ectodermal oesophagus or stomodaeum, at the bottom of which is the blastopore.
The blastopore, bl, has The extent of the shell and closed; the anus will subcommencing upgrowth of the sequently perforate the cormantle-skirt is indicated by responding area.
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 blastopore has open to the exterior by two simple closed.
The little mass of hypoblast or enteric cell-mass now enlarges, but remains connected with the cicatrix of the blastopore or orifice of invagination by a stalk, the rectal peduncle.
The orifice of invagination (blastopore) narrows, and we now have a two-cell-layered sac - the gastrula.
The cavity communicating with the blastopore and lined by the endoderm is the archenteron.
The blastopore, together with the whole embryo, now elongates.
The blastopore then closes along the middle portion of its extent, which corresponds with the later developed foot.
At the same time the stomodaeum, or oral invagination, forms around; the anterior remnant of the blastopore, and the proctodaeum, or anal invagination, forms around the posterior remnant of the blastopore.
There are, however, variations in regard to the relation of the blastopore to the mouth and to the anus which are probably modifications of the original process described above.
Derm or blastopore, (bl).
- In this development the blastopore is not elongated; it persists as the anus.
The blastopore closes, a new mouth and a new anus subsequently arising.
Segmentation is total and at first regular, and is followed by invagination, the blastopore passing to the position of the future mouth.
After invagination is completed, the embryo begins to elongate, the blastopore becomes narrower, and the dorsal wall of the gastrula loses its convexity, and becomes flattened to form the dorsal plate, the outer layer of which is the primordium of the neurochord and the inner layer the primordium of the notochord.
Ova large, with much yolk and thick membrane, like those of Australasian species; embryos with slit-like blastopore and of very different ages in the same uterus, probably born all the year round.
Meanwhile the narrow middle part of the blastopore has closed by a fusion of its lips, so that the blastopore is represented by two openings, the future mouth and anus.
The nervous system is formed as an annular thickening of ectoderm passing in front of the mouth and behind the anus, and lying on each side of the blastopore along the lines of the somites.
B, Older gastrula stage, ventral view, showing elongated blastopore and primitive streak.
C, Ventral view of embryo with three pairs of mesoblastic somites, dumb-bell shaped blastopore and primitive s t r e a k.
D, Ventral view of embryo, in which the blastopore has completely closed in its middle portion.
The gastrula thus formed has a large blastopore, which is at first posterior but afterwards gradually moves towards the anterior end of the ventral surface.
The blastopore generally occupies a position corresponding to the posterior end of the body.
The mesoblast of the cephalic (naupliar) region probably arises in connexion with the lips of the blastopore and consists of loosely-connected cells or mesenchyme.
Io, B) in the gastrula stage, and the orifice of invagination or blastopore, which persists, is situated at the hinder pole.
After a time the planula fixes itself by the anterior pole, with the blastopore uppermost.