MAMMALIA, 479 
annie Cavity. 
Villi of Serosa, 
! 
i 
Serosa. 
Cc 
Yolk-sac. 
Fluid in 
Yolk-sac, 
Villus of Serosa. 
Embryo. 
Allantois. Extra- 
embryonic 
Allantoic Villus. €celom. 
~Yolk-sac 
Villus. 
Prokalymma. 
Epiblast is white, Mesoblast black, Hypoblast dotted. 
—the blastocyst becomes two-layered or diploblastic through- 
out. Both epiblast and hypoblast form thin-walled spheres, 
with a disc or cap of cells at the blastoporic pole. The 
hypoblast remains for the present in this condition, but the 
epiblast divides in a variety of ways to give rise to the 
