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, 



ca vi ty 



'esotheliutn 

 ocardiury) 



m esocardiurn 



Myocardium 

 esoth eLium) 



f>erica.rd. 

 Cavity 



[. Diagrams showing the two anlagen of the heart and their union to form a single 

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

 the ventral mesocardium has disappeared (see text). 



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

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

 :he 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 



