SEGMENTATION OF THE OVUM OF THE SHEEP. 253 



A delay in the accomplishment of the formation of the 

 amnion until after the complete attachment of the edges of the 

 true epiblast to the trophoblast subsequent to its rupture would 

 cause a condition as in the pig or sheep, when the first starting 

 of the amnion seems to be due to the bending up of the edges 

 of the embryonic area, but the full completion of the amnion 

 to be due to other causes, such as the pressure of the enclosed 

 fluids. Thus a state analogous to the Sauropsida is attained. 

 In this case (pig), as I have shown in my figs. Su^, Suj, PL 18, 

 the true amnion is only partly formed by the true epiblast. 

 If this is a correct view, it is interesting to note that there is 

 not the same tendency for the edges of the amnion to fuse in this 

 case, pig (also sheep), as where the free edges of the epiblast 

 coalesce. 



It seems not improbable that in the rabbit this latter process 

 has gone even further. It differs much in its formation from 

 that of the pig. It arises much later, and is practically formed 

 entirely from the hinder fold. 



It is much to be regretted that we have not yet a detailed 

 account of the development of the monotreme blastoderm. 

 Semen's account of the segmentation of the ovum does not 

 extend as far as the separation of the true hypoblast as a 

 permanent layer. There is no trace of an archenteron in his 

 oldest stage on pi. ix of his work. 



I shall not attempt to reconcile this hypothesis with the 

 description of the opossum given by Selenka. The develop- 

 ment of this animal is very unlike that of any placental 

 mammal. Although it has what at first sight seems to be a 

 typical blastodermic vesicle (v. Selenka [51], pi. xix, figs. 1, 5), 

 this, nevertheless, differs from all forms hitherto observed by 

 the absence of a Rauber layer over the formative epiblast. It 

 is clear from this fact, and from the presence of a large " seg- 

 mentation cavity " which never disappears, the evident " blasto- 

 pore," and the peculiar origin of the entoderm, that the 

 Marsupials in their development differ more widely from the 

 placental mammals than the members of the latter group differ 

 among themselves. Selenka's account is by no means com- 



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