28o 



CHORDATE ANATOMY 



myocardium. The right and left halves of the heart begin as longitudinal 

 folds of the splanchnic mesoderm. Between these mesodermal folds 

 and the adjacent endoderm, scattered mesenchyme cells appear, and soon 

 become arranged as a thin-walled endothelial tube in each of the folds. 



The paired mesodermal folds with their enclosed endothelial tubes 

 arise before the ventral wall of the pharynx is formed. The union of the 

 two halves occurs in correlation with the formation of the floor of the 

 pharynx from the endoderm. Successive stages in the process are shown 

 in Fig. 253. 



c d" 



Fig. 253. — Sections cut transversely through the cardiac region of pig embryos of 

 various ages to show the origin of the heart from paired primordia. A, 5-somite 

 embryo; B, 7-somite embryo; C, lo-somite embryo; D, 13 somite embryo. (Projection 

 diagrams X50, from series in the Carnegie Collection.) (From Patten's "Embryology 

 of the Pig.") 



Soon after the tubular heart forms below the pharynx, its wall becomes 

 three-layered by proliferation of cells from the outer or epimyocardial 

 layer. In this way, a thick muscular layer is formed between the endo- 

 thelial lining and the outer serosa. By the union of the visceral layer 

 of mesoderm above and below the heart, dorsal and ventral mesocardia 

 are formed in a manner resembling the formation of mesenteries in relation 

 to the intestine. When the human embryo is a month old, the ventral 

 mesocardial' membrane disappears and the right and left halves of the 

 pericardial cavity are thus brought into direct connexion with one another. 

 By the formation of septum transversum and diaphragm, the pericardial 

 cavity becomes separated from the abdominal cavity. 



