AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 711 



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 contractile 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 which I have just described, it became obvious that this 

 theory would have to be abandoned, and that some other expla- 

 nation 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 differentia- 

 tion 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 Ccelenterata. 



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 differ- 

 entiation of the nervous system may very probably have com- 

 menced, in the first instance, from a specialization 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. 



