194 



YOLK-SACK. 



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 



AA 



S.CaV. 



sx. 



L.of 



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



THE THIRD DAY OF INCUBATION. 



H. heart; A A. 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; R.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.F". 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. 



