36 G. H. Parker 



The views of Kleinenberg and the Hertwigs, as this brief survey 

 shows both contain the common element of simultaneous and 

 inter-related differentiation of nervous and muscular elements. 

 As contrasted with this aspect of the question Claus ('78) and 

 Chun ('80) claimed an independent origin for these two types of 

 tissue and that their connection was secondary. The ground for 

 this opinion, at least as maintained by Chun, is chiefly the condi- 

 tion found in vertebrates where in ontogeny it is very probable 

 that nerve and muscle are independently differentiated and 

 secondarily united. Thus this opinion gets its support from highly 

 specialized rather than from primitive metazoans. 



The view as to the origin of the nervous system, or better of the 

 neuromuscular mechanism, to which the study of the activities of 

 sponges has led me, is in strong contrast with the opinions that 

 have already been expressed. The fact that sponges have an 

 organized musculature, though they show no evidence of nervous 

 organs, leads me to the conclusion that nerve and muscle have not 

 differentiated simultaneously, but that muscular tissue has pre- 

 ceded nervous tissue in order of evolution. The condition in 

 sponges is absolutely contrary to the statement of Kleinenberg 

 ('72, p. 23) that there are no animals with muscles and without 

 nerves, nor is it consistent with the view of the Hertwigs ('78, p. 

 165) and their followers that these two kinds of tissue differentiate 

 simultaneously. Muscular tissue unassociated with anything 

 that can reasonably be called nervous tissue certainly occurs in 

 these primitive metazoans, and muscle of this kind directly stimu- 

 lated, i. e., without the necessary intervention of other cells, is in 

 my opinion the initial stage in the growth of the neuromuscular 

 mechanism. The next step in this process is, I believe, that 

 realized in most coelenterates, i. e., a muscular mechanism to 

 which has been added certain receptive cells, sense cells^ that serve 

 as delicate organs for bringing the muscles into action. This 

 step is the first step in the differentiation of true nervous tissue, 

 though it is the second in the growth of the neuromuscular mech- 

 anism as a whole. At this point my view is in strong contrast 

 with that of Claus and Chun who, as already stated, have main- 

 tained that nerve and muscle arose independently. This I do 



