226 



THE DEVELOPMENT OF THE CHICK 



sides may be approximately in contact, and the cavity of the 

 yolk-sac is thus broken up into numerous connecting compart- 

 ments filled with yolk. The outer wall of the yolk-sac is smooth 

 and not involved in the folds. The beginning of the folds of the 

 yolk-sac may be found at the time of appearance of the vascular 

 area of the blastoderm, and they develop 'pari passu, with the 

 vessels of the yolk-sac (Fig. 131). 



Fig. 131 shows the appearance of the folds at the stage of 

 twelve somites. It is a view of the blastoderm from below, 



m/P. 





Fig. 131. — Septa of the yolk-sac as seen on 



the lower surface of the blastoderm at the 



stage of 12 s. (After Hans Virchow.) 



m. R., Marginal ridge of entoderm overly- 

 ing the sinus terminalis. 



drawn as an opaque object, and it shows the incipient folds of 

 the yolk-sac in an arrangement that corresponds roughly, but 

 not accurately, with that of the blood-islands, which lie in large 

 part in the bases of the folds. The site of the vena terminalis 

 is marked by a circular fold of the entoderm. The folds of the 

 yolk-sac thus coincide in their distribution with the vascular area 

 and are so limited at all times, being absent in the vitelline area. 

 There is thus a close connection between the vitelline blood- 



