THE FCETAL MEMBRANES OF MAMMALS. 



229 



yolk-sac only which is turned toward the embryo. There is developed in it 

 a system of capillaries, which ends abruptly in a marginal vein. The other 



half of the yolk-sac is without vessels, and is everywhere firmly united with the 

 serosa. When, after the resorption of its contents, the yolk-sac commences to 



shrivel, it begins to take on a mushroom-like form (fig. 133 ds\ owing to the 



folding in of the vascular half (fd) against the non. vascular part (ed"), which 



is fused with the serosa (*/<.). It remains united with the umbilicus of the 



embryo by means of an elon- 

 gated intestinal stalk (or 



vitelline duct), which is com- 

 parable to the stalk of the 



mushroom. 



The space () which is 



produced in the blastodermic 



vesicle by the shrinking of 



the yolk-sac does not become 



filled out by compensating 



growths of the amnion (a) 



and allantois (al\ both of 



which remain small. There- 

 fore a large amount of fluid 



collects between the separate 



foetal membranes. The space 



filled with fluid is none other 



than the extra-embryonic part 



of the body-cavity, which in 



the Babbit, as in no other 



Mammal, is highly developed. 



The allantois (al) hangs freely 



in this space as a stalked 



vesicle, a part of its surface 



having applied itself to that 



portion of the serosa (*/*) 



which is not united with the 



yolk-sac, and which is circum- 

 scribed by the sinus termi- 

 nalis (s). It is gradually 



metamorphosed into an organ of nutrition for the embryo, the placenta 

 inasmuch as it receives a rich supply of blood through the vessels of the 

 allantois, the umbilical vessels. 



Subsequently the remaining surface of the blastodermic vesicle, over which 

 the umbilical vessels do not extend, also becomes vascular. This is due to the 

 fact that the albuminous fluid still contained in the mushroom-like yolk-sac 

 becomes entirely absorbed, and that consequently its outer non-vascular and 

 inner, invaginated vascular walls come to lie on each other and to fuse into 

 a single membrane. In this manner the blastodermic vesicle in the Kabbit 

 becomes provided with blood on its entire surface, but from two different 

 sides the placental portion from the vessels of the allantois, and the larger 

 part of the surface from the degenerating vitelline vessels. 



In regard to the formation of the amnion in the Eabbit, upon which VAN 

 BENEDEN ET JULIN have made very thorough investigations, it is to be added 



Pig. 133. Diagrammatic longitudinal section through 

 the ovum of a Rabbit at an advanced stage of 

 pregnancy, after BISCHOFF. 



e, Embryo ; a, amnion ; u, urachus ; al, allantois with 

 blood-vessels ; sh, subzonal membrane ; pi, villi of the 

 placenta ; fd, vascular layer of the yolk-sac ; ed, ento- 

 blast of the yolk-sac ; ed', ed", inner and outer lamellae 

 of the entoblast which lines the flattened cavity of the 

 yolk-sac ; ds, cavity of the yolk-sac ; st, sinus termi- 

 nalis ; r, the space between amnion, allaiitois, and 

 yolk-sac that is filled with fluid. 



