THE r(ETAL MEMBRANES OF MAMMALS. 



231 



which was cut open in the process of preparation. The allantois (al) 

 is attached to the very end of the intestinal canal by means of the 

 attenuated stalk-like urachus. 



Up to this stage the correspondence in the development of the embry- 

 onic membranes in Mammals, Birds, and Reptiles is clear. But from 

 now on the course of development in the Mammalia becomes more 

 and more divergent, since one portion of the embryonic membranes 



Fig. 134. Embryo Dog of 25 days, extended and seen from in front. Magnified 25 diameters. 

 After BISCHOFF. 



d, Intestine ; ds, yolk-sac ; al, allantois, urinary sac ; un, primitive kidney ; I, the two lobes of 

 the liver, with the lumen of the omphalomesenteric vein between them ; ve, he, anterior and 

 posterior appendages ; h, heart ; m, mouth ; an, eye ; g, olfactory pit. 



enters into closer relations with the mucous membrane of the uterus, 

 and is thus converted into an organ of nutrition for the embryo. In 

 this manner a compensation is provided for the loss of the yolk. 



The interesting adaptations for intra-uterine nutrition they have 

 been studied especially by the English anatomist TURNER in a 

 series of profound comparative-embryological works present very 

 great differences in the separate orders of Mammalia : sometimes 

 they are of a simple kind, at other times they are more com- 



