708 ADDRESS TO THE DEPARTMENT OF ANATOMY 



of special sense, especially those of vision, and the central 

 nervous system. 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, however, 

 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 pari passn with the central 

 nervous system. It is probable that in the simplest organisms 

 the whole body is sensitive to light, but that with the appearance 

 of pigment-cells in certain parts of the body, the sensitiveness 

 to light became localised 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 

 differentiated 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 pro- 

 cesses from similar cells placed in other parts of the body. The 

 cells with such processes then became removed from the surface, 

 forming a deeper layer of the epidermis below the sensitive cells 

 of the organ of vision. With these cells they remained connected 

 by protoplasmic filaments, and thus they came to form a thick- 

 ening 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 stimuli so received to other parts of 

 the body. Such a thickening would obviously be the rudiment 

 of a central nervous system, and it is easy to see by what steps 

 it might become gradually larger and more important, and might 

 gradually travel inwards, remaining connected with the sense 

 organ at the surface by protoplasmic filaments, which would then 

 constitute nerves. The rudimentary eye would at first merely con- 

 sist partly of cells sensitive to light, and partly of optical structures 



