Development of the Second Day 16 1 



ment as the head-fold. As will be seen by 

 examination of Figs. 37 and 38, the space be- 

 tween the two layers of the amnion (dotted in 



FIG. 55. TRANSVERSE SECTION THROUGH A CHICK EMBRYO, TO 

 SHOW A SLIGHTLY LATER STAGE IN THE DEVELOPMENT OF THE 



HEART THAN IS SHOWN IN FlG. 48. (After Minot.) 



Md, wall of medullary tube, nek, notochord. mstk, mesothelium. PA, 

 pharynx, pro. am, tip of pro-amnion. en.ht, endothelial heart, m.ht, muscular 

 heart. 



Fig. 37) is continuous with the body or pleuro- 

 peritoneal cavity. 



The allantois may make its appearance be- 

 fore the close of this day, but it will be better 

 to defer its description until the following day's 

 development is discussed. 



