METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 4! 



tubes, is now at the posterior or sinus end of the heart. Conse- 

 quently the death gradient in the heart now extends from the 

 sinus towards the arterial end of the heart. In most of these 

 early stages of the heart there is also still persistent more or less 

 of a reverse gradient from the arterial end of the heart posteriorly. 

 The extent of this differs in different hearts. In some it is very 

 pronounced, in others scarcely evident. It seems probable that 

 there is some growth in length at the arterial end of the heart. 



The disintegration of a heart of a seven somite chick is shown 

 in Figs. 4 to 8. Disintegration begins in the center of the 

 posterior wall of the sinus, extends backward along the vitelline 

 veins, forward along the heart towards the arterial end. At the 

 arterial end there is a slight reverse gradient as shown in Figs. 

 6 to 8. Of four hearts of this age examined three were like the 

 one illustrated while in the fourth the reverse gradient from the 

 arterial end towards the sinus end was considerably more ex- 

 tensive. 



At eight somite conditions in the heart are about as at seven 

 somites, the reverse gradient being generally a little more pro- 

 nounced. The typical course of disintegration at eight somites 

 is illustrated in Figs. 9 to II. Disintegration begins in the 

 posterior wall of the sinus and progresses anteriorly along the 

 heart. It then is initiated at the arterial end and proceeds 

 towards the sinus end, meeting the other disintegration at about 

 the middle of the heart. Of nine hearts of this stage examined, 

 the reverse gradient was slight in three, extended about half way 

 along the heart in four cases, and was entirely absent in one case. 

 In one case there was hardly any difference in time of dis- 

 integration along the heart tube. 



The heart of the nine somite stage is similar to the preceding 

 but differs in one important particular. The disintegration 

 commonly proceeds faster along the right side of the heart than 

 the left. This is illustrated in the figures, numbers 12 to 16. 

 Of nine hearts examined the right side was more susceptible than 

 the left in five cases, in three cases the two sides were about 

 equally susceptible, and in one case, the left side was more 

 susceptible. In eight of them a reverse gradient from the 

 arterial end towards the middle of the heart was present to a 



