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 lacunz in the epiblastic 
outer wall—the trophoblast with its preliminary pathfinding villi. | 
Fic. 401.—View of embryo, with its foetal membranes. 
—After Kennel. 
am., Amnion proper; @., dwindled yolk-sac; a/., allantois; aZ., 
subzonal membrane; z., 2’., villi. Outside the subzonal mem- 
brane there is the delicate ectodermic trophoblast (s.ci.). 
(4) 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 
laver 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- 
