TRANSACTIONS OF SECTION D. — DEPT. ANATOMY AND PHYSIOLOGY. 643 



the neryous system of Vertebrates, and some of these cells are characterised by send- 

 ing 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, 

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



There is every reason to think that tlie network just described is not only con- 

 tinuous 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 epithelium of the 

 skin and that of the digestive tract, but is especially concentrated in the disc-like 

 region between the mouth and tentacles. The above observations have tlirown a 

 very clear light on the characters of the nervous system at an early stage of its 

 evolution, but they leave unanswered the questions (1) how the nervous network 

 first arose, and (2) how its fibres became continuous with muscles. It is probable 

 that the nervous network took its origin from processes of the sense-cells. The 

 processes of the ditterent 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 be- 

 fore 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 contractile processes was a rudimentary nerve. This in- 

 genious theory explained completely the fact of nerves being continuous with 

 muscles ; but on the further discoveries being made which I have 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 the protoplasm 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 in 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 contractile and sensory, and the differentiation of the nervous system 

 may very probably have commenced, in the first instance, from a specialisation in 

 the function of part of a network formed of neuro-muscular prolongations of 

 epithelial cells. A simultaneous differentiation of other parts of the network into 

 muscular fibres may have led to the continuity at present obtaining between nerves 

 and muscles. 



Local differentiations of the nervous network, which was no doubt distributed 

 over the whole body, took place on the formation of organs of special sense, and 

 such differentiations gave rise to the formation of a central nervous system. The 

 central nervous system was at first continuous with the epidermis, but became 

 separated from it and travelled inwards. Ganglion-cells took their origin from 

 sensory epithelial cells, provided with prolongations, continuous with the nervous 

 network. Such epithelial cells gradually lost their epithelial character, and finally 

 became completely detached from the epidermis. 



Nerves, such as we find them in the higher types, originated from special 

 differentiations of the nervous network, radiating from the parts of the central 

 nervous system. 



Suchj briefly, is the present state of our knowledge as to the genesis of the 



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