NERVOUS SYSTEM. 331 



especially of optic organs, are often formed from the same part of the 

 primitive epidermis. Thus the retina of the Vertebrate eye is formed 

 from the two lateral lobes of the primitive fore-brain. 



The same is true for the compound eyes of some Crustacea. The 

 supraoesophageal ganglia of these animals are formed in the embryo 

 from two thickened patches of the epiblast of the procephalic lobes. 

 These thickened patches become gradually detached from the surface, 

 remaining covered by a layer of epidermi?. They then constitute the 

 supraoesophageal ganglia; but they form not only the ganglia, but 

 also the retinulae of the eye — the parts in fact which correspond to 

 the rods and cones in our own retina. The accessory parts of these 

 organs of special sense, viz. the crystalline lens of the Vertebrate eye, 

 and the corneal lenses and crystalline cones of the Crustacean eye, 

 are independently formed from the epiblast after the separation of 

 the part which becomes the central nervous system. 



In the Acraspedote Medusae the rudimentary central nervous 

 system has the form of isolated rings, composed of sense-cells pro- 

 longed into nervous fibres, surrounding the stalks of tentacle-like 

 organs, at the ends of which are placed the sense-organs. 



This close connection between certain organs of special sense and 

 ganglia is probably to be explained by supposing that the two sets 

 of structures actually originated pari passu. 



We may picture the process as being somewhat as follows : — 

 It is probable that in simple ancestral organisms the whole body was 

 sensitive to light, but that with the appearance of pigment-cells in certain 

 pai'ts of the body, the sensitiveness to light became localised to the ai'eas 

 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 connectiou with nervous processes 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 the latter cells they 

 remained connected by protoplasmic filaments, and thus they came to 

 form a thickening of the epidermis iinderneath 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 

 is in fact very similar to the rudimentary ganglia of the Acraspeda mentioned 

 above. It is easy to see by what steps it might become 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 rudimentaiy eye would at first merely consist of 



