THE EARLY DEVELOPMENT OF THE CHICK 255 



little mesoderm is proliferated from the anterior part of the 

 head process, so that in front of this, the blastoderm is for a 

 considerable time composed of only the two primary layers. 



Surface views of entire blastoderms now show considerable 

 modifications in the structure and relations of the primary 

 pellucid and opaque areas. The outline of the elongated pel- 

 lucid area becomes irregular; as a whole it is divisible into a 

 posterior darker region where all three layers are present, and an 

 anterior lighter area, crescentic in form, composed of ectoderm 

 andendoderm only (Fig. 93, B). Later the irregular anterior 

 border of the mesoderm can be seen to advance along the sides 

 of the area pellucida, in front of the level of the head process 

 (Fig. 93, C). Finally the mesoderm extends completely around 

 the margin of the pellucid area, leaving only a small area 

 directly in front of the head process in the two layered condition; 

 this area is known as the proamnion (Fig. 93, D). 



Meanwhile the area opaca has been undergoing very marked 

 modifications. This has broadened rapidly, and in its lateral 

 and posterior regions, where the mesoderm has extended more 

 widely over the germ wall, it has assumed an irregularly mottled 

 appearance. Sections show this to be due to the formation of 

 differentiated cell groups known as blood islands, the beginning 

 of the vascular system. The presence of these blood islands 

 marks that portion of the opaque area known as the area vascu- 

 losa (Fig. 93, C, D). This appears first immediately behind the 

 embryo, but rapidly spreads laterally and anteriorly. Per- 

 ipherally it becomes bounded by a definite blood vessel known 

 as the sinus terminalis. Beyond this the opaque area is formed 

 only of ectoderm and endoderm, extending widely over the 

 yolk and known as the area vitellina. The blood islands are 

 formed of compact cell masses scattered through the deeper 

 portion of the germ wall. The cells composing them have 

 apparently, though not certainly, differentiated in situ; they 

 become covered superficially by a layer of scattered germ wall 

 cells, which comes to be known later as ccelomic "mesoderm" 

 (Fig. 95). While this layer, and the blood islands, early be- 

 come directly continuous with the mesoderm of the pellucid 



