THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



223 



which extends longitudinally for some distance in the cervical region of the 

 embryo. The mesothelial layers of opposite sides meet dorsal and ventral 

 to the endothelial tube, forming the dorsal and ventral mesocardium (Fig. 194). 

 In the meantime the cephalic end of the tube has united with the arterial system, 



Priy, 



f>erica.rd. 



Cavity 



Eqdoca.rd.lunj 

 (Endot/jeLiury) 



Fig. 194. — Diagrams showing the two anlagen of the heart and their union to form a single 

 structure; made from camera lucida tracings of transverse sections of chick embryos. In C 

 the ventral mesocardium has disappeared (see text). 



and the caudal end with the venous system; and in a short time the dorsal and 

 ventral mesocardia disappear and leave the heart suspended by its two ends in 

 the primitive pericardial cavity. The conditions at this point may be sum- 

 marized thus : The heart is a double- walled tube — the inner wall composed of 

 IS 



