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 blastopore would however be situated dorsally, a position which it does not occupy in any gastrula type so far dealt with.
The small cells become the epiblast, and at the blastopore they curl inwards (fig.
At a stage slightly before the closure of the yolk-blastopore the mantle is formed as a slight prominence at the blastodermic pole of the egg, and even at this early stage is marked by the presence of chromatophores.
The blastopore becomes nearly obliterated, but whether it gives rise to the mouth, which is formed in the same place, has not been determined.
When the blastopore remains permanently open the outer part of the oesophagus grows as a prominent ridge round the opening.
In all the oligochaetous types, with the exception of Euaxes, where the blastopore closes completely, the blastopore becomes, or coincides with the mouth.
When the blastopore remains permanently open at the oral region the stomodaeum is formed as an epiblastic wall round its opening.
The blastopore is always situated opposite the original formative pole.
A small blastopore is left by the small cells on the ventral surface, which communicates with an otherwise closed and ciliated cavity which is formed between the two rows of hypoblast cells.
The blastopore now closes up, but its position in relation to the parts of the embryo has not been made out.
The blastopore then closes, but in the position it occupied the epiblast becomes thickened to form the rudiment of the vestibule, which at this stage constitutes a disc marked off by a shallow groove from the remainder of the body.
A longitudinal section of the embryo at a slightly later stage, when the blastopore has become quite narrowed, is represented in fig.
In some instances the blastopore assumes before closing a very narrow slit-like form, and would seem to extend along the future ventral region of the body from the mouth to the anus.
D) the blastopore completely closes, but its position is marked by a shallow pit (m) where the stomodaeum is eventually formed.
By this time the embryo has assumed an oval form, and the blastopore is situated at the pole of the long axis of the oval where the cephalic enlargement is eventually formed.
Bobretzky, the closed blastopore may coincide in position with the mouth in some instances (Nassa, &c.
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.
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 blastopore now closes along the middle part of its course, which coincides in position with the future "foot.
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.
C, Diblastula of an Opisthobranch (Polycera) with elongated blastopore oi.
In this development the blastopore is not elongated; it persists as the anus.
The rectal peduncle or pedicle of invagination; its attachment to the ectoderm is coincident with the hindmost extremity of the elongated blastopore of fig.
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.
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.
Figure 9 gives a similar diagram of a later stage, but here the blastopore is closed.
By a comparison of the figures of frog and fowl the student will easily perceive the complete correspondence of the position of this with the blastopore of the frog.
There is no yolk here to substitute epiboly (Section 9) for invagination, nor to obliterate the archenteron and the blastopore through its pressure.
That the blastopore does become specialized as a larval organ is obvious in those cases in which it becomes transformed into the single opening with which some larvae are, for a time at least, alone provided, e.
It is no doubt the impress of some remarkable larval condition of the blastopore of a stage of evolution now long past.
In Ceratodus the open part of the blastopore is enclosed by the medullary folds, as in Lepidosiren, and probably persists as the anus, the portion of the folds around the anus undergoing atrophy (Semon, Zool.
In the Vertebrata the behaviour of the blastopore (anus of Rusconi) is also variable in a very remarkable manner.
The fact that theblastopore remains open at the hind end of the medullary plate explains to a certain extent the peculiar relation which always exists in the embryo between the hind end of the neural and alimentary canals.
The best case to examine is that of Peripatus capensis, in which the blastopore is at first a long slit, and gives rise to both the mouth and the anus of the adult.
In Urodeles the blastopore persists as anus, so far as is known, but the relation to the medullary folds has not been noticed.
The remains of the original blastopore are present as the mouth, placed between the second pair of mesoblastic somites, and the anus placed on the concavity of the commencing flexure of the hind part of the body.
The blastopore has completely closed in its middle portion, and given rise to two openings, the future mouth and anus.
From one end of the blastopore there is continued an opaque band.
At the edge of the blastoderm which represents the blastopore mouth of Amphioxus all the layers become fused together in the anamniotic vertebrates.
The anterior dilatation of the posterior open region of the blastoporewe shall call the embryonic anus.
The second section of the blastopore in Elasmobranchii or yolk blastopore is, I believe, partly represented by the primitive streak.
The blastopore is elongated and slightly narrowed in the middle.
When it has become very small the attachment of the larva takes place by the flat surface where the blastopore is situated.
The continuity of the epiblast and hypoblast at the hind end of the embryo in the bird and the mammal is a rudiment of the continuity of these layers at the dorsal lip of the blastopore in Elasmobranchii, Amphibia, &c.
Distance between front end of body and front end of blastopore .
The median parts of the lips of the blastopore have come together preparatory to the complete fusion by which the blastopore becomes divided into two parts.
This we consider to be the posterior end of the blastopore of the embryo.
The embryonic mouth and anus are derived from the respective ends of the original blastopore, the middle part of the blastopore closing up.
The medullary folds are seen closely apposed in the median line; hindward, however, they are still separate, and through this opening the blastopore may yet be seen.
In the next stage the closure of the blastopore is rapidly becoming completed; in front of it stretches the widened and elongated form of the embryo.
The medullary folds have completely fused in the median line, and the embryo is coming to acquire a ridge-like prominence; optic vesicles and primitive segments are apparent, and the blastopore appears to persist as the anus.
The rim of the blastoderm has become thickened as the 'germ-ring'; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embryo.
In the earlier stage the dorsal lip of the blastopore is crescent-like; in the later the blastopore acquires its oblong outline, through which the yolk material is apparent; its conditions may later be compared to those of a Ganoid.
It is even probable that what Kowalevsky describes as the carrying of the blastopore to the dorsal side is really the commencement of the formation of the neural canal, the walls of which are continuous with the lips of the blastopore.
The forms in which the position of the blastopore in relation to the mouth or anus is not known are marked with an asterisk.
The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described.
The blastodermic vesicle is therefore formed of three areas, (1) the embryonic area with three layers: this area is placed where the blastopore was originally situated.
In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo; and in C it forms the structure known as the primitive streak.
The succeeding changes commence with the appearance of a narrow cavity between the epiblast and hypoblast, which extends so as completely to separate these two layers except in the region adjoining the original site of the blastopore (fig.
Van Beneden regards it as probable that the blastopore is situated somewhat excentrically in relation to the area of attachment of the hypoblastic mass to the epiblast.
C and D the exposed surface of yolk has become greatly diminished; and an obvious blastopore is thus established.
In Amphioxus, however, the blastopore is at first placed exactly at the hind end of the body, though later it passes up just on to the dorsal side (vide p.
The presence of this blastopore seems to render it clear that the blastopore discovered by Ed.
In the simplest types of Ascidians the series of phenomena is almost the same, but the blastopore assumes a more definitely dorsal position.
This type ofblastopore is characteristic of the Mollusca generally, of the Polyzoa, of the Nematelminthes, and very possibly of the Chaetopoda and Discophora.
The position of the blastopore is the same as in other types, viz.
The circularblastopore in the frog in later years often went by the name of the anus of Rusconi.
In other respects Sedgwick's speculations link on more closely to the Gastræa theory, for one of his main contentions is that the blastopore or Urmund is homologous throughout at least the three metameric phyla.
Why, for instance, should the blastopore so often appear as a long slit, closing by concrescence, unless this had been the original method of its formation in remote Coelenterate ancestors?
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.
Round the blastoporehollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm.
In the hydropolyp the blastopore of the embryo forms the adult mouth situated at the extremity of the hypostome, and the ectoderm and endoderm meet at this point.
Formation of archenteron and blastopore may, however, be deferred till a later stage (actinula or after).
The blastopore is formed as a secondary perforation at one spot, in free-swimming forms at the hinder pole.
In consequence of this the cleft of the blastopore lay between the normal blastopore-lip and the everted border of the other lip.
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 above list will hopefully give you a few useful examples demonstrating the appropriate usage of "blastopore" in a variety of sentences. We hope that you will now be able to make sentences using this word.
