METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 4.} 



regards their death gradients to the eleven somite heart, Figs. 

 22 to 26. In all a total of sixteen hearts of ages from eleven 

 through fifteen somites were studied. In all of these the right 

 side was more susceptible than the left, usually markedly so. 

 In twelve cases, the gradient on the right side was of the type 

 shown in Figs. 22 to 26, that is, disintegration extended from 

 the middle of the right side anteriorly and posteriorly. In the 

 other four the course of disintegration was similar to that shown 

 in Figs. 17 to 21. 



After the stage of fourteen somites the high susceptibility of 

 the middle of the right side of the heart gradually lessens and has 

 completely disappeared by sixteen somites, sometimes earlier. 

 In hearts of embryos of sixteen to over twenty somites there is 

 always present a simple gradient from the sinus to the arterial 

 end of the heart. At this time of course the heart is beating 

 regularly. It is interesting to note that the disintegration of 

 the sinus always begins exactly at the places where the beat is 

 initiated. The disintegration of a heart of a sixteen somite 

 chick is shown in Figs. 27 to 31. The place of initiation of the 

 beat is indicated in Fig. 27 by arrows. Ten hearts of chicks of 

 sixteen to eighteen somites were observed and the course of dis- 

 integration was the same in all of them. 



Conditions also continue the same from these stages through 

 about the second day of development. Figs. 32 to 35 show the 

 disintegration of a heart of a 20 somite chick. As before the 

 sinus begins to disintegrate at the place where the beat originates 

 and from there disintegration proceeds along the heart tube 

 towards the arterial end where there is a slight reverse gradient. 

 From the sinus death progresses caudally along the vitelline 

 veins. Conditions remain the same up to about 25 somites, or 

 the end of the second day of development. 



From the second to the third day there is noticeable a greatly 

 increased susceptibility in the ventricular region. The auricular 

 evaginations also exhibit heightened susceptibility. Figs. 36 to 

 39 illustrate the disintegration of a heart of a thirty somite chick, 

 about sixty hours of incubation. Disintegration begins at the 

 tip of the ventricular bend and from there proceeds anteriorly 

 along the bulbo-conus arteriosus and caudally towards the 



