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BULLETIN OP THE UNITED STATES PISH COMMISSION. 
eggs, while the latter two arrived at their conckisious from a study of the Salmonidce i; 
(Schwarz also worked ou the Pike). I have foiiud that in regard to the point men- ;; 
tinned the former investigators were right — the cavity of the vesicle originally lies ’ i 
between the entoderm and the periblast. ■ 
The essential agreement, however, between the accounts of Henneguy and Schwarz 
leads one to believe that in the Salmonidce the development of the vesicle may have ^ 
suffered secondary modification. Whereas in the Bass the vesicle arises by a process ; 
of folding, in the Salmonidce its development may be construed as the hollowing out • , 
of a solid thickening. After the vesicle has become a closed sac its further develop- ; 
ment and relations to the postanal gut have been correctly described by Schwarz. 
Significance of the vesicle. — The significance of the vesicle is linked with the inter- ! 
pretation of the gastrula, which the vesicle on its side elucidates. In regard to gas- I 
ti ulation I am in thorough accord with Ziegler (48), and therefore regard the space (in 
great part virtual) between the entoderm and the periblast as the archenteron. The ■ ' 
entoderm represents the dorsal hypoblast of the gastrula, the yolk and periblast the 
ventral hypoblast, and it is from the dorsal hypoblast alone that the alimentary canal 
is formed. The alimentary canal is formed by a process of folding, and Kupffer’s I 
vesicle, as the terminal part of the (postanal) gut, follows the same method. After the 
gut has been once folded off, the homology of the vesicle with the postanal vesicle of 
Selachians (instituted by Balfour in his text-book) is obvious. In each group the 
vesicle forms the dilated extremity of the postanal gut, and receives, or would receive 
if it existed, the ueurenteric canal. But I think the homology is just as evident in the 
early stages of Kupffer’s vesicle, as soon as it is recognized that the space between “[ 
the entoderm and the periblast is the archenteron. In Fig. 65, PI. xcvi, Kupffer’s rf i 
vesicle, Ic. -i)., represents tlm dilated terminal portion of the archenteron, while in Fig. ' 
88, PI. xoviii, it is the posterior end of a gut which has been folded off from the arch- 
enteron. The postanal vesicle of Selachians represents both. It forms the end of a [ 
gut produced in great part by folding, and it unquestionably represents the terminal ; 
portion of the archenteron. I 
But if Kupffer’s vesicle in its early stages (Fig. 65) indicates that the terminal por- ; 
tiou of the archenteron was primitively dilated, we naturally inquire both .for the cause ] 
and for a corresponding phenomenon in the ontogeny of those animals in which the i 
archenteron is bodily transformed into the permanent gut. As to the latter point, it i 
would seem very common in the Amphibia for the archenteron to be thus dilated (see j 
Morgan’s figures, 32, and Gotte’s figure, Balfour, T. B., vol. 2, i). 105). The existence 
of such a dilatation in the enteron of jirimitive Chordata is further made probable by, 
and receives an explanation from, the relation of the neurenteric canal to the blasto- 1 
pore. i 
Morgan (32) has shown that in Amblgstoma, after the neurenteric canal has been I 
established by the closure of the upper part of the blastopore, the lower part of the 
blastopore remains as a common opening for the rectum and neural tube (i)ersisting as« 
the permanent anus). This condition he believes existed in adult early Chordata. * 
Cunningham (8) advances a similar suggestion, and I see no reason for rejecting the 
idea. In the hypothetical animals which once existed with this arrangement of parts j 
it is highly probable that the extreme end of the rectum into which the neurenteric | 
canal opened was dilated into a kind of cloaca. And it is this cloaca which is repre- 
sented in the ontogeny of vertebrates by the terminal dilatation of the postanal gut. j 
