THE F(ETAL MEMBRANES OF THE VERTEBRATES. 183 



way as I do. Similarly, Wilson and Hill, in their latest paper 

 on the development of the duck-bill, give us figures of sections 

 which make it probable that the distinction between tropho- 

 blast and formative cells holds good here, even thougli the 

 development of yolk has obliterated the sharp outlines of the 

 process. 



Again, in reptiles and birds traces of the larval layer have, 

 in later years, been unmistakably noticed. Schauinsland, 

 Mitsukuri, and Mehnert were among the foremost to coutri- 

 bute facts in thio direction, although at the same time they 

 failed to see the essential points of comparison with the 

 mammals. This failing on their part is all the more explicable 

 as the bird's egg, which has always served as the prototype 

 even of mammalian development, does not clearly bring out 

 the fundamental distinction that exists between trophoblast 

 and formative matter o£ the embi-yo. 



The gradual obliteration of this distinction may, perhaps, 

 be ascribed to the fact that in these sauropsids, as in the 

 ornithodelphia, a shell has developed, which naturally tends 

 to I'elegate any outer larval layer to the pension list. 



Concerning the yolk accumulation in the sauropsidian egg, 

 there is no trouble at all to suppose that the vesicular blasto- 

 cyst of an early viviparous ancestor has gradually become 

 yolk-laden. The contrary assumption, found in the hand- 

 books, that the mammalian egg, while totally losing its yolk, 

 has yet preserved the identical developmental features as the 

 sauropsid, is, in reality, much more difficult to reconcile with 

 sound evolutionary principles. 



We have seen that a simple clue to our understanding of 

 the complicated foetal envelopes of the sauropsids and the 

 mammalia is the assumption of a simple larval layer, one cell 

 thick, among the invertebrate ancestors. 



We must be ready to admit that this one factor has un- 

 doubtedly given rise to an endless number of variations and 

 modifications in those innumerable families, genera, and 

 species which have come and have gone, ever since the time 

 when viviparity and terrestrial life became an established fact 



