428 CHORDATA. 
Fig. 308.—TuHr EvoLuTion oF THE Fa@:TAL MEMBRANES 
OF VERTEBRATA. 
‘Body-wall, future Serosa, 
Yolk, Yolk-sac Wall. 
Embryo. 
we’ Amniotic Fold. 
\ Extra- 
Alimentary Canal. 
Yolk in Yolk-sac. 
Hypoblast of 
Yolk-sac. 
Extra-embryonic 
Ceelom. 
Mesoblast of - 
. Yolk-sac. 
§ § Mesoblast. ~~ 
5 Epiblast. ~ ga 
wn Yolk-sac. 
Embryo. D 
iniotic 
Amniotic Canal. E Allantois. 
Cavity. , on g 
‘a "a 
ss Oe 
Ped a 
= 6 
68 
aJ 
3 
tad 
moog 
& 
° 
£ 
v 
¢ nN 
° 
~) 
vo 
n 
de 
Yolk-sac. Yolk-sac. 
A, Stage of the Frog with only small Yolk-sac; B, Stage of the Skate ; 
>, Stage of Developing Amnion; D, Stage as in many Reptilia, the amnion and serosa are 
not completely separated (cf also MonotremaTA); E, A Typical Sauropsid. 
Epiblast is represented white, mesoblast black and hypoblast dotted. 
