EMBRYO AND EMBRYONIC MEMBRANES 



227 



vessels and the folds of the yolk-sac, which will be considered 

 more fully bej'ond. 



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- 



FiG. 132. — Part of the interior of the yolk-sac 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, constituting 

 the germ-wall, and in the outer vitelline zone we come to the 

 periblast. 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- 



