826 



PHYLUM CHORDATA : CLASS MAMMALIA 



The three modes of embryonic nutrition are as 

 follows : — 



(a) At first the maternal blood bathes the lacunas in the ectodermal 



outer wall — the trophoblast with its preliminary pathfinding 

 villi. 



(b) An efficient yolk-sac placenta functions for a time, but decreases 



and shrivels as the final allantoidean placenta develops. The 

 maternal blood in the spaces of the outer layer of the mucous 



Fig. 497- — View of embryo, with its foetal membranes. 



— After Kennel. 



am., Amnion proper ; d., dwindled yolk-sac ; al., allantois ; al'., 

 subzonal membrane ; z., /., villi. Outside the subzonal membrane 

 there is the delicate ectodermic trophoblast {s.ch.). 



layer of the uterus bathes the trophoblast. Thus it comes into 

 indirect connection with the vascular villi from the region where 

 the yolk-sac wall unites with the diplotrophoblast. This yolk- 

 sac placenta is well seen in Insectivora, Chiroptera, Rodentia, 

 the horse, etc., and seems to be to some extent developed in all 

 Mammals (except Monotremes) as yet examined, 

 (c) The final placenta is allantoidean. 



In the above description the yolk-sac placenta has been emphasised 

 on account of its comparative importance, but it must be clearly under- 

 stood that the allantoic placenta is often the only one well developed, 

 and is always of supreme importance in reference to the nutrition of 

 the embryo. 



