THE YOLK-SAC. 



701 



Fiir. 512. 





T)ut in the higher animals, as the greater part of the epiblast and 

 Bomatop eure laver of the mesoWast comes to be detached from e 

 ^urfHce of the yolk by the expansion of the amnion and allantois, the 

 hypobas^^ layer of the mesobhast are alone the 



Xanent constituents of the wall of the yolk-sac ;. -d hiwgh he^e 

 last the membrane of the yolk-sac is continuous with the wall ot the 

 intestine in the vitello-intestmal aperture. 



The yolk-sac is the seat of the first circulation of the blood m the 



omphalo-mesenteric yessels of its ^^s^^^lf /^•«^' . ^J*^ J^ .^'Jl^^e 

 animals especially these yessels spread at a later period oyer the who e 

 surface of the yolk in the membrane which forms the sac. Ihe food- 



