Reactions of Sponges 37 



not believe possible, for I agree entirely with Samassa ('92) when 

 he declares that a nervous mechanism without muscles or other 

 effectors is inconceivable. In my opinion nervous tissue has 

 differentiated not independently of muscle, as claimed by Claus 

 and by Chun, but in most intimate relations with it and as a 

 more effective means of bringing it into action than direct stim- 

 ulatiorf is. Primitive muscles, then, as independent effectors, 

 were centers around which the beginnings of nervous differentia- 

 tion probablv occurred, in that certain peripheral cells came to be 

 specialized as receptors for stimuli and excitors of muscular activ- 

 ity, a condition now realized in coelenterates. 



From this standpoint it must be clear that the histogenesis of 

 primitive nervous tissue involves cells that are in contact with 

 the exterior on the one hand and with muscular tissue on the other. 

 The conditions realize almost perfectly the requirements of the 

 well-known theory of neurogenesis advocated by Hensen ('64), 

 and I, therefore, believe that this theory is a truthful portrayal 

 of primitive neurogenesis. I do not admit, however, that it pre- 

 sents a correct picture of the histogenesis of vertebrate nerves. 

 In this problem the evidence seems to me to be strongly in favor 

 of the initial separateness of nerve and muscle and their secondary 

 union, an operation which in my opinion is a coenogenetic modi- 

 fication of the primitive process. But whether the axis-cylinders 

 of vertebrate nerve-fibers are outgrowth of neuroblasts or not, is 

 a question that has no direct bearing on the one herein discussed, 

 the differentiation of the primitive nervous system. Such a primi- 

 tive nervous system, essentially receptive in character, is, how- 

 ever, merely the beginning of that structure which in the higher 

 metazoans is designated as nervous. This primitive nervous 

 system is not in any appropriate sense to be called centralized. 

 Its diffuse character, from an anatomical as well as from a physio- 

 logical standpoint, is well known, and only after the nervous 

 structures have become concentrated either in the periphera 

 lepithelium, as in some worms, or on separation from this epithe- 

 lium, as in the higher metazoans, is a condition arrived at which 

 necessitates the formation of true nerves, and allows the establish- 

 ment of common paths (Sherrington, '06), a condition which 



