222 



PARKER— ORIGIN AND SIGNIFICANCE OF 



[April 21, 



taneoiis stinnilation of many such elements. It was Kleinenberg's 

 opinion that these neuromuscular cells (Fig. i, B) divided (C) and 

 thus gave rise to the nerve-cells and muscle-cells (D) of the higher 

 animals. In fact he declared that the nervous and muscular systems 

 of these animals were thus to be traced back to the single type of 

 cell, the neuromuscular cell, which morphologically and physiolog- 



FiG. I. Diagram to illustrate Kleinenberg's theory of the neuromusculai" 

 cell. A, epithelial stage; B, neuromuscular cell; C, neuromuscular cell partially 

 divided; D, nerve-cell and muscle-cell of ccelenterate stage. 



ically represented the beginnings of both. But Kleinenberg's neuro- 

 muscular cells were subsequently shown by the Hertwigs to be merely 

 epitheliomuscular cells and no intermediate stage between them and 

 the differentiated neuromuscular mechanism of higher forms was 

 ever discovered. Hence this hypothesis, too. has been largely aban- 

 doned. 



Some years later, in 1878, the Hertwigs published an account of 

 the neuromuscular mechanism in coelentcrates, an account which even 

 at the present time is accepted as authoritative by most students of 

 the subject. In this account they described the sensory cells, the 

 ganglionic cells, and the muscular cells of the ccelenterates, and 

 maintained that these elements arose not by the division of single 

 cells, as implied in Kleinenberg's hypothesis, but that each element 

 was differentiated from a separate epithelial cell (Fig. 2) and yet 

 in such a way that during differentiation all these elements were 

 physiologically interdependent. This h}pothesis of the simultaneous 

 differentiation of nerve and muscle is the current opinion among 

 biologists today. 



