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 (sh). It remains united with the umbilicus of the 

 embryo by mean-s of an elon- 

 .gated intestinal stalk (or 

 vitelline duct), which is com- 

 parable to the stalk of the 

 mushroom. 



The space (r) 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 Rabbit, 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 (sk) 

 which is not united with the 

 yolk-sac, and which is circum- 

 scribed by the sinus termi- 



Fig. 133. Diagrammatic longitudinal section through 

 the ovum of a Babbit at an advanced stage of 

 pregnancy, after BISGHOFF. 



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

 blood-vessels ; sh, subzonal membrane ; pl t 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, alLantous, and 

 yolk-sac that is filled with fluid. 



nalis (st). It is gradually 



metamorphosed into an organ of nutrition for the embryo, the placenta (pi), 

 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 Rabbit 

 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 Rabbit, upon which VAN 

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



