EMBRYO AND EMBRYONIC MEMBRANES 227 
vessels and the folds of the yolk-sac, which will be considered 
more fully beyond. 
The interior of the yolk-sac is lined with entoderm which 
differs in its structure in different regions: In the area pellucida 
the cells are flattened; in the vascular zone of the area opaca 
are found the columnar cells with swollen ends described pre- 
viously. After the third or fourth day these are found filled 
with yellow fatty droplets, which give a yellow tone to the interior 
of the living yolk-sac, and which are so abundant in later stages 
as to render the layer perfectly opaque. These cells do not con- 
Fic. 152. — Part of the interior of the yolk-sae of a 
duck at the time of hatchng. In the upper part of 
the figure the septa are seen from the side showing 
the stomata. In the lower part they are seen on 
edge. Note the sinuous course of the arteries along 
the free edges of some of the septa. (After H. 
Virchow.) 
tain entire yolk-granules; apparently, then, the yolk-granules are 
digested before absorption in this region. In the region of the 
inner zone of the vitelline area, the entoderm is composed of 
several layers of large cells containing yolk-granules, and in 
the outer vitelline zone we come to the germinal wall. The 
germinal wall and inner zone of the vitelline area represent 
the formative region of the yolk-sac epithelium in the manner 
already described (Chap. V). 
Blood-vessels of the Yolk-sac. The development of the circu- 
