2l6 



EMBRYOLOGICAL TYPES 



during the embryonic life. At the end of this period the 

 allantois is not drawn up into the embryo, but nipped off at 

 the umbilicus and left behind. 



The relations of the embryonic membranes are simple 

 to make out if it is remembered that mesoderm underlies 

 ectoderm, and mesoderm overlies endoderm. All the 

 boundaries of any particular cavity are continuous when traced 



oat y c 



Fig. i 02. — Gallus : diagram showing the final relations of the embryonic 

 membranes. (From Jenkinson, after Duval and Lillie.) 



ach, air-chamber ; all, allantois the outer wall of which is closely- 

 pressed against the shell-membrane ; amc, amniotic cavity which com- 

 municates with the (as) albumen sac through the reopened (sac) sero- 

 amniotic connexion ; ast, allantoic stalk ; c, extra-embryonic ccelom ; sh y 

 shell ; y, yolk in the yolk-sac which still communicates with the albumen- 

 sac ; x, point at which the yolk-sac will eventually close. 



right round, and are formed from one germ-layer. They are 

 consistent in their arrangement, so that an endodermal cavity 

 (gut, yolk-sac or allantois) cannot communicate with a cavity 

 lined by mesoderm (ccelom), or with one lined by ectoderm 

 (amnion, or atrium of Amphioxus). 



Vascular System. — At an early stage, the blood-islands of 

 the area vasculosa become connected up with a pair of anterior 

 vitelline veins which run towards the embryo from each side, 



