6o Vertebrate Embryology 



met beneath the pharynx forms the dorsal mesentery of 

 the heart. The mesoderm around the tube continues to 

 thicken, and forms later the musculature of the heart. 



" At first the heart has also a ventral mesentery formed 

 by the union of the walls of the coelomic cavities below 

 it (Fig. 25), but later the mesentery is in part absorbed 

 and the coelomic cavities become continuous below from 

 side to side, forming the pericardial chamber. The 

 outer layer of somatic mesoderm gives rise to the peri- 

 cardium itself. 



" The tubular heart is attached at its posterior end to 

 the liver and anteriorly to the wall of the pharynx. It 

 becomes free ventrally and also dorsally along the middle 

 of its course, and owing to an increase in length is bent 

 on itself into an m -shaped tube (Fig. 14)." ' 



A series of transverse constrictions now 

 gives indication of the division of the heart 

 into the various chambers, though there are, 

 for a time, no actual partitions between the 

 different regions. A septum is finally formed 

 which divides the single auricle into right and 

 left halves, and by the time of metamorphosis 

 the heart has practically the adult structure. 



Without distinguishing between the exter- 

 nal and internal gills, the larval circulation is, 

 briefly, as follows : " The venous blood, re- 

 turned from the body at large, enters the pos- 

 terior end of the heart, or sinus venosus ; from 



' Morgan. 



