AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 709 



constituting the lens, which would throw an image of external 

 objects upon it, and so convert the whole structure into a true 

 organ of vision. It has thus come about that, in the develop- 

 ment of the individual, the retina or sensitive part of the eye 

 is first formed in connection with the central nervous system, 

 while the lenses of the eye are independently evolved from the 

 epidermis at a later period. 



The general features of the origin of the nervous system 

 which have so far been made out by means of the study of 

 embryology are the following : 



(1) That the nervous system of the higher Metazoa has 

 been developed in the course of a long series of generations 

 by a gradual process of differentiation of parts of the epidermis, 



(2) That part of the central nervous system of many forms 

 arose as a local collection of nerve-cells in the epidermis, in the 

 neighbourhood of rudimentary organs of vision. 



(3) That ganglion cells have been evolved from simple 

 epithelial cells of the epidermis. 



(4) That the primitive nerves were outgrowths of the original 

 ganglion cells ; and that the nerves of the higher forms are 

 formed as outgrowths of the central nervous system. 



The points on which embryology has not yet thrown a satis- 

 factory light are : 



(1) The steps by which the protoplasmic processes, from 

 the primitive epidermic cells, became united together so as to 

 form a network of nerve-fibres, placing the various parts of the 

 body in nervous communication. 



(2) The process by which nerves became connected with 

 muscles, so that a stimulus received by a nerve-cell could be 

 communicated to and cause a contraction in a muscle. 



Recent investigations on the anatomy of the Ccelenterata, 

 especially of jelly-fish and sea-anemones, have thrown some 

 light on these points, although there is left much that is still 

 obscure. 



In our own country Mr Romaines has conducted some in- 

 teresting physiological experiments on these forms; and Professor 

 Schafer has made some important histological investigations 

 upon them. In Germany a series of interesting researches have 

 also been made on them by Professors Kleinenberg, Claus and 



