384 DEVELOPMENT OF ELASMOBRANCH FISHES. 



General considerations. One point of general anatomy upon 

 which my observations throw considerable light, is the primitive 

 origin of nerves. So long as it was admitted that the spinal 

 and cerebral nerves developed in the embryo independently 

 of the central nervous system, their mode of origin always 

 presented to my mind considerable difficulties. It never ap- 

 peared clear how it was possible for a state of things to have 

 arisen in which the central nervous system as well as the 

 peripheral terminations of nerves, whether motor or sensory, 

 were formed independently of each other ; while between them 

 a third structure was developed, which, growing out either 

 towards the centre or towards the periphery, ultimately brought 

 the two into connection. That such a condition could be a 

 primitive one seemed scarcely possible. 



Still more remarkable did it appear, on the supposition that 

 the primitive mode of formation of these parts was represented 

 in the developmental history of Vertebrates, that we should find 

 similar structural elements in the central and in the peripheral 

 nervous systems. The central nervous system arises from the 

 epiblast, and yet contains precisely similar nerve-cells and nerve- 

 fibres to the peripheral nervous system, which, when derived 

 from the mesoblast, was necessarily supposed to have an origin 

 completely different from that of the central nervous system. 

 Both of these difficulties are to a great extent removed by the 

 facts of the development of these parts in Elasmobranchs. 



It is possible to suppose that in their primitive differentia- 

 tion contractile and sensory systems may, as in Hydra 1 , have 

 been developed from the protoplasm of even the same cell. 

 As the sensory and motor systems became more complicated, the 

 sensory portion of a cell would become separated by an in- 

 creasing interval from the muscular part of a cell, and the two 

 parts of a cell would only be connected by a long protoplasmic 

 process. When such a condition as that was reached, the 

 sensory portion of the cell would be called a ganglion-cell or 

 terminal sensory organ, the connecting process a nerve, and the 

 contractile portion of the cell a muscle-cell. When these organs 

 were in this condition, it might not impossibly happen for the 

 general developmental growth which tended to separate the 



1 Kleincnbcrg Hydra. 



