12 BIOLOGY AND SOCIAL PROBLEMS 



into two opposing camps, the neurogenists 

 who believe that the heart-beat is essentially 

 nervous in its origin and the myogenists who 

 hold that it is muscular in source. But quite 

 aside from the way in which this question 

 may be settled, one point seems to be well es- 

 tablished, and that is that in the embryo of 

 such a vertebrate as the chick, the heart be- 

 gins to beat before any nervous tissue what- 

 ever can be discovered in it. At this stage, 

 then, the heart-beat must be purely muscular 

 and the nervous complications such as they 

 are in the adult must be of later origin. 



The peculiar condition seen in the embry- 

 onic heart suggests that in primitive animals 

 muscle may have preceded nerve in its evo- 

 lution, and in fact confirmation of this view 

 seems to be presented by sponges. In these 

 animals the presence of muscular tissue was 

 long ago recognized, and recent studies on 

 their activities have shown that sponges are 

 enabled by means of this tissue to close their 

 pores and other apertures, and change slightly 

 the form of their bodies. They are not known 

 to possess any nervous tissues whatever, and 

 the responses which they exhibit, like those 

 of the embryonic vertebrate heart, are appar- 



