428 



CHORD AT A. 



Fig. 308. — The Evolution of the Fcetal Membranes 

 OF Vertebrata. 



Yolk. 



Y l>ody-wall, future Serosa. 

 Yolk-sac Wall. 



Yolk in Yolk-sac. 



Hypoblast of 



Yolk-sac. 



Extra-embryonic 



Coelom. 



Mesoblast of- 



. Yolk-sac. 



% ( Mesoblast. 



2 I Epibl; 



Alimentary Canal, 



Embryo. 



Amniotic Fold. 

 Extra- 

 embryonic 

 Coelom. 



Yolk-sac. 



Embryo. 



E Allantois. 



/ 



Yolk-5 



Yolk-sac. 



A, Stage of the Frog with only small Yolk-sac ; B, Stage of the Skate ; 



C, Stage of Developing Amnion ; D, Stage as in many Reptilia, the amnion and serosa are 



not completely separated (c/T also Monotremata) ; E, A Typical Sauropsid. 



Epiblast is represented white, mesoblast black and hypoblast dotted. 



