370 



COMPARATIVE ANATOMY 



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

 in Figs. 306 and 307. 



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- 



B30 HOURS O 32 HOURS T^ 38 HOURS T7' 40 HOUF 



10 somites ^ 12 somites -'--' '6 som.tes JJ/ 1! somit, 



rL 44 HOURS 



H 



K 



76 HOURS 

 M somites 



100 HOURS 

 4S somites 



Fig. 308. — Ventral views of the heart of chick embryos at successive stages to show 

 its changes of shape and its regional differentiation. Abbreviations: a. v., constriction 

 between atrium and ventricle; i.v., interventricular groove. (Prom Patten's " Embryol- 

 ogy of the Chick.") 



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. By the time the 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. (Fig. 311) 



