MAMMALIA, 



Amniotic Cavity. 



479 



Embryo, 



Villi of Serosa. 



Fluid in 



Yolk-sac. „_ v\ 



Villus of Serosa. 



Embryo. 



Allantois. 

 Allantoic Villus. 



Extra- 

 ,^^ ^embryonic 

 ' " ^^ Coelom. 



Fluid in Yolk-sac. 



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 

 tiypoblast remains for the present in this condition, but the 

 spiblast divides in a variety of ways to give rise to the 



