142 Vertebrate Embryology 



partly to the slight increase in the depth of 

 the head-fold and partly to the slight lifting 

 of the head above the surrounding blasto- 

 derm. The increase in depth of the head- 

 fold causes a corresponding increase in the 



Fjg. 48. — Transverse section of a chick embryo with seven 

 segments; to show the beginning of the formation of the 

 HEART. (After Minot.) 



Md^ medullary groove; Ec^ ectoderm; Tnes^ mesenchyma; Ant, veSy amnio- 

 cardiac vesicle; Ph^ pharynx; rnsth^ mesothelium; Endo, cells to form the rudi- 

 ment of the endothelial heart. 



length of the fore-gut (Fig. 50), which, in 

 cross section (Fig. 48, P/i), is a shallow cavity, 

 in a dorso-ventral direction ; but is broad and 

 crescentic, from side to side, with the convex 

 side of the crescent downwards. 



