EARLY STAGES OF DEVELOPMENT OF THE MOUSE. 75 



to Assheton's theory; an objection wliich he himself has 

 seen. If the trophoblast is hypoblastic; then the false 

 amnion of the Mammalia cannot possibly be homologous 

 with that of the Sauropsida, and the character by which 

 these two groups have up till the present been distinguished 

 from the Anamnia has no morphological value whatever. 

 Assheton pleads that in the Sauropsida it is from the hypo- 

 blast that the embryo draws its first nourishment, and that 

 as the trophoblast performs a similar function in the Mam- 

 malia, it may therefore be regarded as hypoblastic; I can 

 only say that to do this seems to me to substitute analogy 

 for homology, in short to abandon the morphological stand- 

 point altogether; and as such the argument stands self- 

 condemned. 



In conclusion, speaking for myself, I believe that all 

 attempts to institute homologies between the mammalian 

 blastocyst as such, and the segmented ova of other Verte- 

 brata, are foredoomed to failure, just because the conditions 

 under which the Mammalian ovum develops are so peculiar, 

 and because we have at present no clear idea of how this 

 ovum came to be derived from that of either an amphibian 

 or reptilian ancestor. The phenomena which we observe in 

 the early ontogeny of the Mammalia are, in my humble 

 opinion, absolutely sui generis; and I believe that the 

 only sure method of arriving at any homologies at all is to 

 take, not the ovum, but the embryo at a time when it 

 possesses an organ, such as an amnion, which can be de- 

 finitely compared with a similar organ in other groups, and 

 from this to argue backwards to an earlier stage. 



Until lately two separate processes have been, so far as 

 concerns the Vertebrata, confused under the term "gastrula- 

 tion." The first is a movement of the vegetative cells to- 

 wards the animal pole, inside the cells of which they may, 

 and do in small-yolked, holoblastic eggs, become included ; 

 the second is a backward and lateral overgrowth and in- 

 growth of animal cells to form a notochordal and mesodermal 

 plate, and possibly the roof of the gut on the inside, and on 



