CONNECTION BETWEEN EMBRYO AND MOTHER. 733 



7. At the area of fixing, the uterine epithelium degenerates, the 

 glands disappear, vascularity increases. The outer part of the 

 modified mucous membrane (or decidua) becomes a spongy 

 tissue, with spaces filled with maternal blood. This maternal 

 blood bathes the trophoblast, which is intermediate between it 

 and the placental villi. 



The three modes of embryonic nutrition are as follows : 



(a) At first the maternal blood bathes the lacunce in the epiblastic 

 outer wall the trophoblast with its preliminary pathfinding villi. 



FlG. 401. View of embryo, with its foetal membranes. 

 After Kennel. 



am. t Amnion proper; d. t dwindled yolk-sac; /., allantois; al '., 

 subzonal membrane ; e. t z'., villi. Outside the subzonal mem- 

 brane there is the delicate ectodermic trophoblast (s.ck.). 



(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 

 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- 



