404 



EVOLUTION OF THE NERVOUS SYSTEM. 



certain epithelial cells, which became at the'same time especially 

 differentiated as sense-cells (figs. 236 and 237). 



FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF CARMARINA 

 HASTATA. (After O. and R. Hertwig.) 



A. Neuro-epithelial sense-cell, c. sense-hair. 



B. Transitional cell between a neuro-epithelial cell and a ganglion -cell. 



(3) These processes gave rise to a subepithelial nervous 

 plexus, in which ganglion-cells, formed from sense-cells which 

 travelled inwards and lost their epithelial character (fig. 237 B), 

 soon formed an important part. 



(4) Local differentiations of the nervous network, which was 

 no doubt distributed over the whole body, took place partly in 

 the formation of organs of special sense, and partly in other 

 ways, and such differentiations gave rise to a central nervous 

 system. The central nervous system was at first continuous 

 with the epidermis, but became separated from it and travelled 

 inwards. 



(5) 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. 



The following points amongst others are still very obscure : 



(1) The steps by which the protoplasmic processes from the primitive 

 epidermic cells became united together so as to form a network of nerve- 

 fibres, placing the various parts of the body in nervous communication. 



(2) The process by which nerves became connected with muscles, so 

 that a stimulus received by a nerve-cell could be communicated to and 

 cause a contraction in a muscle. 



It is probable, as stated in the above summary, that the nervous net- 



