4O LIBBIE H. HYMAN. 



entoderm and ectoderm is semicircular in outline, its concavity 

 directed posteriorly. "The head fold thus produces an internal 

 bay in the entoderm, the beginning of the fore-gut" (Lillie, 

 " Development of the Chick," 2d edition, p. 91). The posteriorly 

 advancing crest of the head fold thus marks the posterior limit 

 of the fore-gut and constitutes the floor of the anterior intestinal 

 portal. 



When embryos of two or three to five or six somites are studied 

 in disintegrating solutions, ventral side up, it is seen that the 

 central part of the semicircular crest of the head fold is the 

 most susceptible and that susceptibility diminishes from the 

 center posteriorly along the margins of the head fold. This is 

 shown in my Figs. I to 3. 



At about the stage of four somites the mesoderm begins to 

 invade the head fold as the so-called amnio-cardiac vesicles. 

 These two cavities push in towards the median line. In the 

 splanchnic wall of each amnio-cardiac vesicle where it lies 

 adjacent to the entoderm of the advancing crest of the head 

 fold, a mesodermal tube appears. These two tubes are as is 

 well known the primordia of the heart. Upon studying embryos 

 at this time, about five somites, the disintegration of this region 

 is the same as earlier. The central part of the crest of the head 

 fold is the most susceptible and the susceptibility diminishes 

 along the posteriorly extending margins of the fold which now 

 contain the heart tubes. The death of this region of a five to 

 six somite embryo is shown in my Figs, i to 3. The metabolic 

 conditions of the heart-forming region at this time are thus in 

 harmony with the fact that the two heart primordia fuse from 

 before backward, a high metabolic rate being essential for the 

 process of fusion, as seen in the preceding paper. 



By about the stage of seven somites the two heart primordia 

 have fused, so that a small heart is present. But the crest of 

 the head fold, now perhaps more correctly designated the floor 

 of the anterior intestinal portal, still retains its high metabolic 

 rate. Consequently after the heart tubes have united the point 

 of high susceptibility and metabolism is still the middle of the 

 crescentic fold which marks the posterior extent of the fore-gut. 

 This region, however, formerly at the anterior end of the heart 



