METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 47 



SUMMARY. 



1. Before the formation of the heart the middle of the crest 

 of the head fold is highly susceptible and the susceptibility 

 decreases along the posteriorly directed margins of this fold 

 which contain the primordia of the heart. This finding is in 

 harmony with the fact that the two heart primordia fuse from 

 in front backward. 



2. After the heart is formed the anterior intestinal portal, 

 now situated at the posterior end of the heart, retains its high 

 susceptibility, and this fact appears to induce a corresponding 

 direction of the gradient in the heart. In the heart at all stages 

 from its first formation (about seven somites) to the latest stage 

 studied (three days) there is a general susceptibility gradient from 

 the sinus towards the arterial end. This gradient appears to 

 originate from the manner of formation and backward progression 

 of the anterior intestinal portal. In early stages of the heart 

 there is usually also present a reverse gradient at the arterial 

 end, due probably to the persistence of conditions stated in i. 



3. There is thus in the heart before it has begun to beat a sin- 

 arterial gradient in metabolic rate. This is probably the cause 

 of the sequence of the heart beat. 



4. In addition to the general sin-arterial gradient, secondary 

 regions of heightened susceptibility appear at times. The most 

 important of these is the increased susceptibility of the right side. 

 This begins at about the nine somite stage and persists until 

 about fifteen somites. It is correlated with the curvature of the 

 heart to the right. Between the second and third day ventricles 

 and auricular evaginations show temporary local increased sus- 

 ceptibility. 



5. Described abnormalities of the heart can be correlated with 

 these findings on differential susceptibility along the heart. 



BIBLIOGRAPHY. 

 Dareste, C. 



'91 Recherches sur la production artificielle dcs monstruosites. Deuxieme 



edition. Paris. 

 Hyman, L. H. 



'18 Suggestions Regarding the Causes of Bioelectric Phenomena. Sci., N.S., 

 Vol. 48, No. 1247. pp. 518-24. 



