68 



Vertebrate Embryology 



It retains its connection with the dorsal aorta 

 for a considerable time, but eventually be- 

 comes separated from it, so that all of the 

 blood that now passes from the bulbus arte- 

 riosus directly to the aorta must pass through 

 the third or systemic arch (Fig. 27, 2). 



FIG. 25. DIAGRAMS TO ILLUSTRATE THE MODE OF DEVELOP- 

 MENT OF THE HEART. 



En^ entoderm. EC, ectoderm. , endothelial lining of the heart. 

 M, muscular wall of heart. Mes, mesoderm. PC, pericardium. PA, 

 pharynx. (Somewhat altered from Morgan.) 



The carotid gland, a characteristic structure 

 in the anatomy of the frog, is formed as an 

 elaboration of the direct communication be- 

 tween the afferent and efferent vessels of the 

 first branchial arch. 



The development of the other blood vessels 

 will be described in connection with the chick, 



