1 90 DEVELOPMENT OF THE SPINAL NERVES 



developed in the embryo independently of the central nervous 

 system, their mode of origin always presented to my mind con- 

 siderable difficulties. 



It never appeared 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 

 in both directions (towards the centre and towards the peri- 

 phery), ultimately brought the two into connexion. 



That such a condition could be a primive 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, if derived, as is usually stated, 

 from the mesoblast, was necessarily supposed to have a com- 

 pletely different origin from the central nervous system. 



Both of these difficulties are to a great extent removed 

 by the facts of the development of these parts in Elasmo- 

 branchs. 



If it be admitted that the spinal roots develop as outgrowths 

 from the central nervous system in Elasmobranch Fishes, the 

 question arises, how far can it be supposed to be possible that in 

 other vertebrates the spinal roots and ganglia develop indepen- 

 dently of the spinal cord, and only subsequently become united 

 with it. 



I have already insisted that this cannot be the primary con- 

 dition ; and though I am of opinion that the origin of the 

 nerves in higher vertebrates ought to be worked over again, yet 

 I do not think it impossible that, by a secondary adaptation, the 

 nerve-roots might develop in the mesoblast 1 . 



1 [May 1 8, 1876. Hensen's observations, as well as those recently made by 

 myself on the chick, render it almost certain that the nerves in all Vertebrates spring 

 from the spinal cord.] 



