1G2 ORIGIN OF NERVES. 



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 be- 

 tween them a third structure was developed, which, growing 

 out either towards the centre or towards the periphery, ulti- 

 mately brought the two into connection. That such a con- 

 dition 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\ 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 

 ganglion-cell and the muscle-cell to be so rapid as to render it 

 impossible for the growth of the connecting nerve to keep pace 

 with it, and that thus the process connecting the ganglion- cell 

 and the muscle-cell might become ruptured. Nevertheless the 

 tendency of the process to grow from the ganglion cell to the 



^ KlcincnbciG Hydra. 



