STUDIES IN MAMMALIAN EMBRYOLOGY. 531 



The different size which the blastocyst of the same degree 

 of development attains in different mammals (extremes being 

 represented e. g. by the rabbit on one side and by the hedgehog 

 on the other) may partly be influenced by the more or less favor- 

 able conditions of nutrition under which the vascular area finds 

 itself placed. In a former publication {' Quart. Journ. Micr. 

 Sci./ 1889) I have described these as particularly favorable 

 in the case of the hedgehog. In short, I have here touched 

 upon several points which have all contributed to an important 

 change in the function and also in the development of the 

 outer wall of the early blastocyst. Now this change will, I 

 presume, have been equally momentous for the development of 

 the inner layer of the didermic blastocyst — the hypoblast. 



And there can hardly be a doubt that the earliest function of 

 the trophoblast, as above hypothetically described, can certainly 

 be rendered more effectual if at the same time the hypoblast 

 follows suit, and constitutes at the earliest possible moment 

 an inner lining to the trophoblastic sac. 



The area vasculosa spreads out between these two mem- 

 branous cell layers. The danger of a slight defect in a mono- 

 dermic expanded blastocyst might be reduced by 50 per cent, 

 if the blastocyst is not monodermic, but didermic. The latter 

 consideration may still further help us to understand a pecu- 

 liarity in the gastrulation of the Mammalia, as compared to 

 that of the Reptilia (lizards [Weldon, Strahl, Hoffmann] ; 

 tortoises [Kupffer, Mitsukuri and Ishikawa], a. o.). The 

 palingenetic phenomenon of infolding at the lip of the blasto- 

 pore, which in the latter is so clear and considerable, and so 

 intimately linked with the formation of a neurenteric canal, is 

 ever so much more obscured in mammals. Now, if the elastic 

 tension of the mammalian blastocyst is a distinctive charac- 

 teristic which has developed in the way that has been above 

 hypothetically sketched, then we can very well understand that 

 an open-mouthed blastopore has come to be more or less oblite- 



too mechauical. Selection will probably not have operated in such a direct 

 way, and the swellings of the maternal tissues are parallel to the increase in 

 size of the ovum; they are certainly not occasioned by it (cf. fig. 12). 

 VOL. XXXI, PART IV. — NEW SEE. N N 



