THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



235 



nerve cells appear in it and before any nerve fibers are connected with it. At least no technic 

 has yet been devised by which it is possible to demonstrate nerve cells in, or fibers connected 

 with it, at the time when it begins to perform its characteristic function. And, furthermore, 

 at the time when the heart begins to beat, no heart muscle cells are developed. This last 

 fact seems to indicate an inherent contractility in the mesothelial cells which form the anlage 

 of the myocardium. 



Sup. vena cava- 



Inf . vena cava 



Right atrium 



Right ventricle _ 



Inf. vena cava 



- Left ventricle 



Fig. 208.— Dorsal half of foetal heart. Bumm, Kottmann's Atlas. 



The Vessels. 



Origin. — It has already been stated at the beginning of this chapter that the 

 heart and blood vessels arise independently, and only secondarily come into 

 close relationship. The heart has its origin inside the embryo; the first vessels 

 and first blood cells, on the other hand, have their origin in the extraembryonic 

 area of the germ layers. In the chick embryo toward the end of the first day of 

 incubation, the peripheral^ part of the area opaca presents a reticulated appear- 

 ance when seen from the surface (Fig. 209) . Sections of the blastoderm show 

 that this appearance is due to thickenings in the mesoderm, and that at this 

 stage there is no ccelom in this region (Fig. 2 10) . The thickenings, however, 

 are situated rather nearer the entodermal side, and after the mesoderm splits 



