161 



Yolk-Sack. The origin of the area opaca has already been de- 

 scribed. It rapidly extends over the yolk underneath the vitelline 

 membrane ; and is composed of epiblast and of the hypoblast of the 

 germinal wall continuous with that of the area pellucida, which on the 

 fourth day takes the form of a more or less complete layer of 

 columnar cells 1 . Between the epiblast and hypoblast there is a layer 

 of mesoblast, which does not extend as far as the two other layers. 

 The yolk is completely surrounded by the seventh day. 



Towards the end of the first day blood-vessels begin to be de- 

 veloped in the inner part of the mesoblast of the area opaca. Their 

 development is completed on the second day ; and the region through 

 which they extend is known as the area vasculosa. The area 



S.T. 



FIG. 125. DIAGRAM OF THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE THIRD 



DAY OF INCUBATION. 



H. heart ; AA. the second, third and fourth aortic arches ; the first has become 

 obliterated in its median portion, but is continued at its proximal end as the external 

 carotid, and at its distal end as the internal carotid ; AO. dorsal aorta; L.Of.A. left 

 vitelline artery; E.Of.A. right vitelline artery; S.T. sinus terminalis; L.Of. left 

 vitelline vein; R.Of.' right vitelline vein; S. V. sinus venosus ; D.C. ductus Cuvieri ; 

 S.Ca.V. superior cardinal vein ; V.Ca. inferior cardinal vein. The veins are marked 

 in outline and the arteries are black. The whole blastoderm has been removed from 

 the egg and is supposed to be viewed from below. Hence the left is seen on the right, 

 and vice versa. 



1 Further investigations are required as to the character of this layer. 

 B. E. II. 11 



