INDEPENDENT EFFECTOBS 55 



propagated as a wave from the venous to the arterial end 

 of this organ. 



The discovery by Eemak in 1844 that the heart muscle 

 of vertebrates contains an abundance of nerve cells has 

 been used as a strong argument in favor of the neuro- 

 genic theory. But it is now generally believed that these 

 cells are merely concerned with modifying a heart-beat 

 which in its origin is myogenic. From this standpoint, 

 therefore, the presence of these nerve cells affords no 

 serious obstacle to the acceptance of the myogenic inter- 

 pretation of the heart-beat, but it does leave the adult 

 vertebrate heart a less clear example of an independent 

 effector than might be desired. 



Although in this respect the adult heart falls short 

 of all that might be wished, the embryonic heart of ver- 

 tebrates is quite otherwise. In the developing chick the 

 heart is the first complicated organ to assume functional 

 activity. It arises after about twenty-three hours of in- 

 cubation and it begins to pulsate about six hours later. 

 At this step in the development of the chick, the stage of 

 ten somites, the neural crests are not yet formed and 

 neuroblasts are not yet differentiated. Hence there is 

 every reason to believe that the heart is absolutely free 

 from possible nervous influence and that its beat must be 

 purely myogenic. Not until some time after the heart 

 has been in action, in the chick on the sixth day of incu- 

 bation, is this organ first invaded by nerve cells, a condi- 

 tion also to be observed in a number of other vertebrates 

 (His, 1893). Hooker (1911) has likewise shown that in 

 frog embryos from which the developing nervous sys- 

 tem has been removed, the heart not only differentiates 

 but eventually beats, a condition that confirms the older 

 results on the chick. Hence the early embryonic heart in 



