642 BEPORT — 1880. 



processes of a nervous nature inwards, which came into connection with nervous 

 processes from similar celk 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 thickening 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_ consist of cells sensitive to light ; at a later period there would be 

 formed optical structures 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 development 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 col- 

 lection 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 satisfactory light are : — 



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

 dermic 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 contrac- 

 tion in a muscle. 



Recent investigations on the anatomj^ of the Ooelenterata, 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. Romanes has conducted some interesting physiological 

 experiments on these forms ; and Professor Schafer has made some important 

 histological investigations upon them. In Germany a series of valuable researches 

 have also been made on this group by Professors Kleinenberg, Glaus and Eimer, 

 and more especially by the brothers Hertwig, of Jena. Careful histological in- 

 vestigations, especially those of the last-named authors, have made us acquainted 

 with the forms of some very primitive types of nervous system. In the common 

 sea-anemones there are, for instance, no organs of special sense, and no definite 

 central nervous system. There are, however, scattered throughout the sldn, 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 

 a fine process which penetrates 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, moreover, 

 present in the network cells of the same character as the multipolar ganglion-cells in 



