232 



THE MESODERMAL DERIVATIVES 



cardial 



one side merges ventrally with that on the other side (except at the 

 extremities of the embryo) to form a horseshoe-shaped cavity sur- 

 rounding the yolk and enteron. The only region where the coelom 

 does not become continuous is the area dorsal to the enteron (gut) 

 or ventral to the notochord. At this region the splanchnic mesoderm 

 comes together to form the dorsal mesentery. This double sheet of 

 splanchnic mesoderm then acts as a suspensory mesentery for the 

 gut, and it continues around to enclose the enteron and the yolk. This 

 lining of the visceral cavity and covering of the gut will become the 

 peritoneal epithelium. 



Just posterior to the pharynx the coelomic space extends into 

 the heart mesenchyme as the pericardial cavity, to be described below. 



During the tail bud stage, for in- 

 stance, one could inject a colored 

 dye into the coelom at the yolk level 

 and this would be carried anteriorly 

 to fill the forming pericardial cavity. 

 The frog develops lung respiration 

 by the time of metamorphosis but it 

 does not develop a diaphragm to 

 separate the coelom further into a 

 pleural and peritoneal cavity as in 

 the case of higher vertebrates. These 

 cavities remain continuous, in the 

 frog, as the pleuro-peritoneal cavity, 

 but the pericardial cavity becomes 

 cut off entirely from the embryonic coelom. This is accomplished by 

 the aid of the developing ductus Cuvieri which helps to form the 

 septum trans versum at about the time of metamorphosis. 



The Heart. 



The heart rudiment develops early from splanchnic mesenchymal 

 cells which grow ventrally toward the mid-line below the pharynx. 

 There they aggregate to form paired spaces (anterior coelomic spaces) 

 at about the tail bud stage. This seems to occur under the inductive 

 influence of the archenteric floor. In the meantime there appear 

 scattered and possibly endodermal cells dorsal to the point of junction 

 of the converging mesodermal masses and ventral to the pharynx. 

 These are known as endothelial cells which become endocardial cells. 



The coelom and its derivatives in 

 the frog. 



