INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 921 



may very probably have commenced, 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 differentia- 

 tion of otlaer 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 difierentiations gave rise to the formation of 

 a central nervous system. The central nervous system was at first 

 continuoiis with the epidermis, but became separated from it, and 

 travelled inwards. Ganglion-cells took their origin from sensory 

 epithelial cells prjovided with prolongations continuous with the 

 nervous network. Such ei)ithelial cells gradually lost their epithelial 

 character, and finally became completely detached from the ejjidermis. 



Kcrves, 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." 



With regard to organs of special sense, Mr, Balfour (in an earlier 

 part of his address) pointed out that it might have been anticipated 

 a priori that organs of special sense would only appear in animals 

 provided with a well-developed central nervous system. This, how- 

 ever, is not the case. Special cells with long delicate hairs, which 

 are undoubtedly highly sensitive structures, are present in animals 

 in which as yet nothing has been found which could be called a 

 central nervous system ; and there is every reason to think that the 

 organs of special sense originated j^ci^'i passu with the central nervous 

 system. It is probable that in the simplest organisms the whole 

 body is sensitive to light, but that with the apijcarauce of pigment- 

 cells in certain parts of the body, the sensitiveness to light became 

 localized to the areas where the pigment-cells were present. Since, 

 however, it was necessary that stimuli received by such organs should 

 be communicated to other parts of the body, some of the epidermic 

 cells in the neighbourhood of the pigment-spots, which were at first 

 only sensitive, in the same manner as other cells of the epidermis, 

 became gradually difiercntiated into special nerve-cells. 



As to the details of this differentiation, embryology does not as 

 yet throw any great light ; but from the study of comparative anatomy 

 there are grounds for thinking that it was somewhat as follows : — 

 Cells placed on the surface sent protoplasmic processes of a nervous 

 nature inwards, which came into connection with nervous processes 

 from siinilar cells placed in other jjarts of tlic body. The cells with 

 such processes then became removed from the surface, forming a deep 

 layer of the epidermis below the sensitive cells of the organ of vision. 

 Witli these cells they remained connected by protoplasmic filaments, 

 and thus they came to form a thickening of the epidermis underneath 

 the organ of vision, the cells of which received their stimuli from 

 those of the organ of vision, and transmitted the stinnili so received 

 to other parts of tlio body. Such a thickening would obviously bo 

 tlic rudiment of a central nervoiis system, and it is easy to see by 

 what steps it might become gradually larger and more important, and 



