H2 THE BIOLOGY OF THE FROG chap. 



and the two sides of the pericardial cavity become continu- 

 ous ; the dorsal connection of the heart disappears at a 

 later period. The visceral layer of pericardium which closely 

 invests the heart becomes reflected upon the sides of the 

 surrounding cavity, where it becomes continuous with the 

 parietal layer, the relations of the two parts being essentially 

 the same as that of the portion of peritoneum surrounding 

 the alimentary canal and that lining the coelom. Owing to 



Fig. 28. — A, B, C, three stages in the development of the heart. E, endo- 

 thelium ; PE, pericardium ; PH, pharynx; W, wall of heart. (After Mor- 

 gan.) 



its increase in length the heart becomes bent in the form of 

 an S ; anteriorly it becomes continued into the truncus 

 arteriosus, which divides into two branches which proceed 

 toward the gills, where they break up into the aortic arches, 

 which distribute branches to the gill filaments. The blood 

 vessels first appear as lacunae or spaces between the cells of 



