METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 45 



of the stimulated condition (Hyman, '18). I have pointed out 

 that the chief difference between a stimulated and unstimulated 

 organ is apparently one of rate of activity. A muscle or gland 

 in a stimulated state does not appear to exhibit phenomena 

 different from those characteristic of it in the unstimulated 

 condition; it simply carries on its particular processes at a 

 faster rate. If this be true then the essential feature of stimu- 

 lation is an increase in the rate of processes in the organ stimu- 

 lated. Granting this premise it follows that any organ whose 

 rate of activity is already sufficiently high will of necessity be 

 automatic. From this point of view the cause of the heart beat 

 may be expressed simply as follows: certain or all of the heart 

 tissue has so high a rate of activity that it functions in the 

 absence of extrinsic stimulation. Now it is not of any conse- 

 quence whether the tissue of the heart which possesses this high 

 rate of activity is muscle or nerve. The state of affairs in this 

 regard differs in different hearts. It is highly probable that in 

 all hearts when they begin to beat in the embryo the automaticity 

 resides in the muscle cells. But the muscle cells may lose their 

 high metabolic rate with age and in that event the aid of the 

 nervous system must be partially or wholly evoked to keep the 

 apparatus going. The nervous system appears to be charac- 

 terized by a high metabolic rate and is thus able to control 

 absolutely or alter the rate of activities of other organs. In the 

 process of ontogeny it comes to exercise more or less complete 

 control over the heart, the degree varying in different animals. 



It remains to consider briefly the teratology of the heart. 

 Dareste in his book ('91) discusses only one type of anomolous 

 heart in the chick, namely, the duplicature of this organ. This 

 condition as he correctly points out results from a failure of the 

 two heart primordia to unite. This union begins at the anterior 

 end of the two heart tubes, a region of high susceptibility as 

 shown in this paper. Depressing conditions would undoubtedly 

 inhibit this union in whole or part resulting in the occurrence of 

 partially or completely doubled hearts. 



On the basis of the susceptibility data presented in this paper 

 one might also expect certain other abnormalities of the heart. 

 Thus I have shown that at an early stage of the heart the right 



