42 LIBBIE H. HYMAN. 



greater or less extent. The greater susceptibility of the right 

 than of the left side of the heart may already be evidenced at 

 the eight somite stage. Its interpretation is considered im- 

 mediately. 



Two points may be emphasized here. First, in these early 

 stages before the heart has begun to beat, there is already present 

 a gradation in susceptibility, i.e., in metabolic rate, from the 

 sinus towards the arterial end of the heart. This gradient 

 appears to originate from the backward growth of the anterior 

 intestinal portal, which, at the time of the formation of the 

 heart, takes place by the fusion from before backward of the 

 splanchnopleuric margins of the head fold. Thus the direction in 

 which the heart beat will pass along the heart seems to be 

 determined by the manner of development of the fore-gut of 

 the embryo. The second point concerns the greater susceptibility 

 of the right side of the heart. This condition undoubtedly fore- 

 shadows the bending of the heart to the right which occurs 

 subsequently. As emphasized in the preceding paper of this 

 series all such bendings are preceded by a higher metabolic rate 

 on the side which is to become the convex surface of the bend. 



At the stage of ten somites the greater susceptibility of the 

 right side is generally very pronounced. The disintegration of 

 a heart of a ten somite chick is shown in Figs. 17 to 21. Dis- 

 integration proceeds from the sinus along the right side, later on 

 the left side. There is generally still present a slight reverse 

 gradient at the arterial end. Thirteen hearts of this stage were 

 examined, of which eleven exhibited this reverse gradient, eight 

 to the extent shown in the figures, three to a greater extent. 



At about the stage of eleven somites the heart begins to bend 

 to the right. This condition is marked by a high susceptibility 

 of the middle of the right side. Disintegration begins as usual 

 in the sinus end of the heart but soon attacks the middle of the 

 right side; from this point it then extends anteriorly and pos- 

 teriorly along the right side, and laterally towards the left side. 

 The disintegration of a heart of eleven somites is shown in Figs. 

 22 to 26. From this time on the reverse gradient at the arterial 

 end is either absent or very slight in extent. 



Hearts of twelve to fifteen somites are generally similar as 



