920 RECORD OP CURRENT RESEARCHES RELATING TO 



cells are characterized by sending a process into the superjacent 

 epithelium. Such cells are obviously epithelial cells in the act of 

 becoming nerve-cells ; and it is probable that the nerve-cells are, iu 

 fact, sense-cells which have travelled inwards and lost their epithelial 

 character. There is every reason to think that the network just 

 described is not only continuous with the sense-cells in the epithelium, 

 but that it is also continuous with epithelial cells which are provided 

 with muscular prolongations. The nervous system thus consists of a 

 network of protoplasmic fibres, continuous on the one hand with 

 sense-cells in the epithelium, and on the other with muscular cells. 

 The nervous network is generally distributed both beneath the epi- 

 thelium of the skin and that of the digestive tract, but is especially 

 concentrated in the disk-like region between the mouth and tentacles. 



The above observations have thrown a very clear light on the 

 characters of the nervous system at an early stage of its evolution, but 

 they leave unanswered the questions how the nervous network first 

 arose, and how its fibres became continuous with muscles. 



It is probable that the nervous network took its origin from pro- 

 cesses of the sense-cells. The processes of the different cells probably 

 first met and then fused together, and becoming more arborescent, 

 finally gave rise to a complicated network. 



The connection between this network and the muscular cells also 

 probably took place by a process of contact and fusion. 



Epithelial cells with muscular processes were discovered by 

 Kleinenberg before epithelial cells with nervous processes were 

 known, and he suggested that the epithelial part of such cells was a 

 sense-organ, and that the connecting part between this and the con- 

 tractile processes was a rudimentary nerve. This ingenious theory 

 explained completely the fact of nerves being continuous with muscles ; 

 but on the further discoveries being made just described, it became 

 obvious that this theory would have to be abandoned, and that some 

 other explanation would have to be given of the continuity between 

 nerves and muscles. The hypothetical explanation just offered is 

 that of fusion. 



It seems very probable that many of the epithelial cells were 

 originally provided with processes, the protoplasm of which, like that 

 of the Protozoa, carried on the functions of nerves and muscles at the 

 same time, and that these processes united amongst themselves into a 

 network. By a process of differentiation parts of this network may 

 have become specially contractile, and other parts may have lost their 

 contractility and become solely nervous. In this way the connection 

 between nerves and muscles might be explained, and this hypothesis 

 fits in very well with the condition of the neuro-muscular system as 

 we find it iu the Coelenterata. 



The nervous system of the higher Metazoa appears then to have 

 originated from a differentiation of some of the superficial epithelial 

 cells of the body, though it is possible that some parts of the system 

 may have been formed by a differentiation of the alimentary epithelium. 

 The cells of the epithelium were most likely at the same time con- 

 tractile and sensory, and the differentiation of the nervous system 



