INTRODUCTION 23 



resented the beginnings of both. But Kleinenberg 's neu- 

 romuscular cells were subsequently shown by the Hert- 

 wigs to be merely epitheliomuscular cells and no inter- 

 mediate stage between them and the differentiated neuro- 

 muscular mechanism of higher forms was ever discov- 

 ered. Hence this hypothesis, too, has been largely aban- 

 doned (Parker, 1911). 



Some years later, in 1878, the Hertwigs published an 

 account of the neuromuscular mechanism in co?lenterates. 

 In this account they described the sensory cells, the gan- 

 glionic cells, and the muscular cells of the ccelenterates, 

 and maintained that these elements arose not by the di- 

 vision of single cells, as stated by Kleinenberg, but that 

 each element was differentiated from a separate epithe- 

 lial cell and yet in such a way that during differentiation 

 all these elements were physiologically interdependent. 

 This hypothesis of the simultaneous differentiation of 

 nerve and muscle, which has been the current opinion 

 among biologists for more than a generation, is not with- 

 out its serious difficulties, for it appears that in the 

 sponges, which are more primitive animals than the 

 ccelenterates, there are muscles of a very simple type but 

 without any associated nerve (Parker, 1910 a). In fact, 

 no nervous tissue of any kind has been definitely identified 

 in sponges. It, therefore, appears that of the two ele- 

 ments, nerve and muscle, the latter may exist indepen- 

 dently of the former and in such a way as to indicate 

 its more primitive character. From this standpoint it 

 seems that the receptor-effector system of the ccelenter- 

 ates was preceded by a simpler state in which only the 

 effector element, muscle, was present and that this element 

 may, therefore, be regarded as the original one in the 

 evolution of the neuromuscular mechanism. Muscle once 



