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 



FIG. 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; mes, mesenchyma; Am. ves, amnio- 

 cardiac vesicle; Ph, pharynx; msth, 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, Pk\ 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. 



