402 EVOLUTION OF THE NERVOUS SYSTEM. 



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 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 thicken- 

 ing 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 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 rudimentary eye would at first merely consist of cells sensitive 

 to light, and of ganglion-cells connected with them ; while at a later period 

 optical structures, constituting a lens capable of throwing an image of 

 external objects upon it, would be developed, and so convert the whole 

 structure into a true organ of vision. It has thus come about that, in the 

 development of the individual, the retina is often first formed in connection 

 with the central nervous system, while the lenses of the eye are indepen- 

 dently evolved from the epidermis at a later period. 



A series of forms of the Ccelenterata and Platyelminthes 

 affords us examples of various stages in the differentiation of a 

 central nervous system 1 . 



In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs 

 of special sense, and no definite central nervous system. There are, however, 

 scattered throughout the skin, and also throughout the lining of the digestive 

 tract, a number of specially modified epithelial cells, which are no doubt 

 delicate organs of sense. They are provided at their free extremity with a 

 long hair, and are prolonged on their inner side into fine processes which 

 penetrate into the deeper part of the epithelial layer of the skin or digestive 

 wall. They eventually join a fine network of protoplasmic fibres which forms 

 a special layer immediately within the epithelium. The fibres of this net- 

 work are no doubt essentially nervous. In addition to fibres there are, 



1 Our knowledge on this subject is especially due to the brothers Hertwig (Nos. 

 320 and 321), Eimer (No. 318), Claus (No. 317), Schiifer (No. 326), and Hubrecht 

 (No. 323). 



