We may distinguish the following series of stages: (I) ovum; (2) cleavage, leading to formation of a blastula; (3) formation of an inner mass or parenchyma, the future endoderm, by immigration or delamination, leading to the so-called parenchymula-stage; (4) formation of an archenteric cavity, the future coelenteron, by a splitting of the internal parenchyma, and of a blastopore, the future mouth, by perforation at one pole, leading to the gastrula-stage; (5) the outgrowth of tentacles round the mouth (blastopore), leading to the actinula-stage; and (6) the actinula becomes the polyp or medusa in the manner described elsewhere (see articles Hydrozoa, POLYP and Medusa).
By tangential division of the cells of the blastoderm, as in Geryonia, or by a mixture of immigration and delamination, as in Hydra, Tubularia, &c. The blastopore is formed as a secondary perforation at one spot, in free-swimming forms at the hinder pole.
Formation of archenteron and blastopore may, however, be deferred till a later stage (actinula or after).
There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely.
The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally.
In such cases the blastopore may entirely close, and both mouth and anus develop as new ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N.
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.
Segmentation is complete, a gastrula is formed, the blastopore closes, the archenteron gives off two coelomic sacs which, as far as is known, are unaffected by the super ficial segmentation of the body that divides the larva into three segments.
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.
The internal structural differences are even more characteristic. In the hydropolyp the blastopore of the embryo forms the adult mouth situated at the extremity of the hypostome, and the ectoderm and FIG.
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.
A, pilidium larva anus by the closure in of nemertine; B, Asplanchnapus the middle of such a slitschematized; C, a ploimal rotifer; like blastopore opening D, trochosphaera female (schematized into a sack-like stomach from Semper); E, veliger larva of mol as is seen in the larvae lusc; F, trochophore larva of annelid.
The blastopore is now more F, Later embryo seen as a transcontracted, d; and cells, me, m, Mouth.
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.
An important fact to note is that the blastopore is included in this overgrowth of epiblast, so that the neural tube remains for some time in open communication with the archenteron by means of a posterior neurenteric canal.
It is thus possible that the neurenteric canal is due to the conjunction of a posterior neuropore with the blastopore, i.e.
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.
That the mouth at the extremity of the hypo c, Visual cone; n, stome represents the persistent blastopore of nucleus; n.
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.
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.