METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 45 



tion of this is that the heart beat originates in the sinus venosus, 

 as has long been known, and is transmitted from the sinus in 

 sequence along the heart tube ; in order that such transmission 

 shall occur, the original impulse must attain a certain strength. 

 As the sinus is the most susceptible part of the heart, its beat is 

 weakened by the action of the toxic solution and the impulse 

 generated in it becomes too feeble to be transmitted along the 

 length of the heart tube. At first it is able to reach as far as the 

 auricle but not to the ventricle and bulbus, which consequently 

 no longer contract while the auricle still continues to beat; sub- 

 sequently the beat becomes too feeble to be transmitted as far as 

 the auricle and the sinus remains beating by itself. In later 

 stages of the heart, the auricle may continue to beat after the 

 sinus has ceased ; it is probable that as development proceeds, the 

 auricle develops some slight degree of independence and auto- 

 maticity of its own, as is well known for the auricles of the 

 lower vertebrates, and being less susceptible to toxic agents than 

 the sinus it may continue to contract after the latter has ceased. 



The heart is by far the most susceptible part of the Fundulus 

 embryo and dies and disintegrates shortly after the embryo is 

 exposed to the ammonia solution. Older hearts are more sus- 

 ceptible than younger ones. This indicates that the metabolic 

 rate of the heart increases during development. 



These observations show that there is a gradation in meta- 

 bolic rate along the heart tube from the sinus to the arterial end 

 of the heart. Such a gradation is in all probability the cause of 

 the sequence of the heart beat. The sequence of the heart beat 

 is generally stated in textbooks of physiology to be due to the 

 fact that the venous end of the heart possesses a more rapid 

 intrinsic rhythm than the other chambers of the heart and con- 

 sequently " sets the pace " for them. This is really only another 

 way of saying that the venous end of the heart has a higher rate 

 of metabolism than the rest of the heart tube, for how could it 

 contract more rapidly than they if such were not the case? 

 Nevertheless it does not seem to have occurred to physiologists 

 that in such a simple gradation in metabolic rate rests the ex- 

 planation of the sequence of the beat. It is a familiar physio- 



