NERVOUS SYSTEM. 401 



The embryological evidence shews that the ganglion-cells of 

 the central part of the nervous system are originally derived 

 from the simple undifferentiated epithelial cells of the surface of 

 the body, while the central nervous system itself has arisen from 

 the concentration of such cells in special tracts. In the Chor- 

 data at any rate the nerves arise as outgrowths of the central 

 organ. 



Another important fact shewn by embryology is that the 

 central nervous system, and percipient portions of the organs of 

 special sense, 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 supracesophageal ganglia of these animals are formed in the 

 embryo from two thickened patches of the epiblast of the pro- 

 cephalic lobes. These thickened patches become gradually 

 detached from the surface, remaining covered by a layer of 

 epidermis. 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 

 prolonged 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 part 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 

 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 



B. III. 26 



