STUDIES IN MAMMALIAN EMBRYOLOGY. 523 



reception of that blood. The genesis of such communications 

 is undoubtedly facilitated in the shrew by the fact that not 

 only the trophoblast is composed of young and newly formed 

 cells, but that the same holds good for the maternal prolifera- 

 tion with the crypts. The spaces which in both are destined 

 for the transport of maternal blood fuse, so to say, in statu 

 nascenti, and this helps to explain how in the shrew the 

 process of fusion can only be traced with so much difficulty, 

 and how the boundary lines between maternal and embryonic 

 proliferations become so very soon untraceable. 



The final reduction of the maternal cryptal tissue to a layer 

 of nuclear nests interspersed between the placentary tissue and 

 the muscularis (see figs. 32 and 54) need not be discussed in 

 detail. The reduction is a gradual one, and partly figured in 

 fig. 86, where the maternal tissue between this cryptal prolifera- 

 tion and the muscularis has not yet developed anew — rearranged 

 itself preparatory to parturition, as it has already done in 

 fig. 54. 



The more deeply stained maternal nuclear nests just referred 

 to are thrown ofi" with the placenta. The way in which the 

 maternal surface regenerates after parturition will not be here 

 discussed, but will be reserved for another paper, in which I 

 will then include comparative considerations with respect to 

 other genera of Insectivora (Erinaceus, Talpa, and Tupaja). 



