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 yclk 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 volk-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- 
Josa. Within the latter area the differentiation is completed. 
