'I ill- HKTAI- Mi UK MAM' 





yolk->ac unly which i- tu. I the em 1 )pe.d in it 



in i'f capillaries. \\ Inch Midi abruptly in a marginal vein. The. other 



half of tho yolk-sac is without ve>srls, and is everywhere tinnly united wit h the 



i When, after t In- resorption of its cont. oik-sac conun* t 



shrivel, it begilM to take on a mushroom-like f.,nn ' - ), Mwini: to the 



folding in of the \a-cidai Irilt (./'</) acr.'iinM the non-va<cul.ir ]-. v.hirh 



-ed with the serosa (\//). It remains united witli the umbilicus of the 

 embryo by m. ans of an elon- 

 il stalk (or 



vitelline duct), which is com- 

 I arable to the stalk of the 

 mushroom. 



The >pace (;) which is 

 produced in the blastodermic 

 vesicle by the shrinking of 

 tho yolk-sac does not become 

 filled out by compensating 

 growths of the amnion (a) 

 and allantois (ul\ both of 

 which ivinain small. There- 

 fore a large amount of fluid 

 collects between the separate 

 foetal membranes. The space 

 filled with fluid is none other 

 than tho 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 

 . a part of its surface 

 having applied itself to that 

 portion of the serosa (sh) 

 which is not united with the 

 yolk-sac, and which is circum- 

 scribed by the sinus termi- 

 nalis ($0- 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 blastodennio vesicle, over which 

 the umbilical ressels 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 fu>e 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 

 FT Ji LIN have made very thorough investigations, it is to be added 



Fig. 133. Diagrammatic longitudinal section through 

 the ovum of a Rabbit at an advanced stage of 

 pregnancy, after BISCUOFF. 



e, Embryo ; a, amnion ; u, urachus ; at, 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 cuter lamella) 

 of the entoblast which lines the flattened cavity of the 

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

 nulls; r, the space between amnion, nllantui.s, an I 

 yolk-sac that is tilled with fluid. 



