90 THE DEVELOPMENT OF THE CHICK 



nect and anastomose with one another, forming a vascular net- 

 work lying between the coelomic mesoblast and the remains of 

 the germ-wall. This network spreads throughout the vascular 

 area, and appears later in the pellucid area, and communicates 

 with the blood-vessels of the embryo (Figs. 44 and 45). In the 

 next chapter we shall consider the manner in which the extension 

 takes place, and the origin of the blood-vessels and blood-cells. 



IV. THE GERM-WALL 



The germ-wall arises, as we have seen, through infiltration 

 of the superficial white yelk by the periblast. These cells mul- 

 tiply and anastomose and form a multinucleated syncytium with 

 the yolk-granules in its meshes. By degrees the protoplasm itself 

 takes up the yolk-granules, which are gradually digested, and the 

 germ-wall thus becomes organized as a coherent layer. It then 

 separates from the underlying yolk. The next period in the 

 history of the germ-wall is its differentiation, which takes place 

 in the vascular area concomitantly with the formation of the blood- 

 islands: a considerable proportion of the protoplasm and nuclei 

 of the germ-wall accumulates at the surface and forms the vascu- 

 lar mesoderm in the manner already described. The part of the 

 germ-wall that remains after the separation of the mesoderm then 

 differentiates into the characteristic entodermal epithelium of the 

 opaque area, which is known as the yolk-sac epithelium (ento- 

 derm) because it is destined to form the lining of the yolk-sac. 



After the formation of the vascular area the term germ-wall 

 must be restricted to the lower layer of the vitelline area, because 

 within the vascular area it has already differentiated into the 

 mesoderm and yolk-sac entoderm. The development of the 

 germ-wall takes place in a centripetal direction; at any period 

 during the overgrowth of the yolk the three stages of the germ- 

 wall may be found in the concentric zones. The first stage, 

 that of periblast, is found in the zone of junction (area vitellina 

 externa); the second stage, that of organization of the germ- 

 wall, is found in the area vitellina interna; and the third stage, 

 that of differentiation, is found at the margin of the area vascu- 

 losa. Within the latter area the differentiation is completed. 



