636 REPRODUCTION. 



one below, next the entoderm ; and a little later (Fig. 186) the division 

 between them is more complete. This cleft represents the future peri- 

 toneal cavity. The external lamina of the mesoderm will afterward 

 supply the voluntary muscles and other tissues of the thoracic and 

 abdominal walls ; while the involuntary muscular layer of the aliment- 

 ary canal is derived from its internal lamina. Outside the body of 

 the embryo the internal lamina increases in thickness, and becomes of 

 great importance in the formation of the blood and blood-vessels. 



The first appearance of vascularity shows itself, early in the second 

 day, in the parts immediately surrounding the area pellucida. Certain 



FIG. 190. 



PORTION OP THE VASCULAR AREA, from the blastoderm of the chick at the fortieth hour of incu- 

 bation, showing islets of blood and embryonic blood-vessels. Vt. Vena terminalis. (Kolliker.) 



spots in the mesoderm of this region assume the form of irregularly- 

 shaped spaces filled with red blood globules, which soon unite with each 

 other in such a way as to form a network of inosculating vessels. The 

 region of the blastoderm occupied by this network is known as the 

 " area vasculosa." There is at first no arrangement of the blood-vessels 

 in trunks and branches, nor any apparent distinction corresponding to 

 that of arteries and veins. All are of nearly the same size, distributed 

 on the same plane and communicating equally with each other ; except 

 that they are surrounded by a large vessel, the " vena terminalis," with 

 which all the adjacent vessels communicate and which thus forms the 

 .exterior limit of the vascular area (Fig. 190). Subsequently a differ- 

 ence shows itself in the extent and rapidity of their growth, some pre- 

 ponderating in size over the others ; and by the end of the third day 

 there is a visible distinction between their trunks, branches, and rami- 

 fications. At the same time the formation of blood-vessels extends 



