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-sae (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, 
Fie. 13: 
the lower surface of the blastoderm at the 
stage of 12s. (After Hans Virchow.) 
m. R., Marginal ridge of entoderm overly- 
ing the sinus terminalis. 
Septa of the yolk-sac as seen on 
drawn as an opaque object, and it shows the incipient folds of 
the yolk-sae 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-sae 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- 
