Development of the Second Day 161 



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 DEVKLOPMENT OF THE 



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



Md, wall of medullary tube. nch. notochord. msth^ mesothelium. /%, 

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



