190 
ME. F. M. BALFOUE ON THE DEVELOPMENT OF 
These statements, which have not been since confirmed, diverge nearly to the same 
extent from my own results as does the ordinary account of the development of these parts. 
Hensen (Virchow’s ‘ Archiv,’ vol. xxxi. 1864) also looks upon the spinal nerves as 
developed from the epiblast, but not as a direct result of his own observations*. 
Without attempting, for the present at least, to explain this divergence, I venture to 
think that the facts which I have just described have distinct bearings upon one or 
two important problems. 
One point of general anatomy upon which they throw considerable light is the primi- 
tive 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 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 periphery), ultimately brought the two into connexion. 
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, if derived, as is 
usually stated, from the mesoblast, was necessarily supposed to have a completely 
different origin from the central nervous system. 
Both of these difficulties are to a great extent removed by the facts of the develop- 
ment of these parts in Elasmobranchs. 
If it be admitted that the spinal roots develop as outgrowths from the central nervous 
system in Elasmobranch Fishes, the question arises, how far it can be supposed to be 
possible that in other vertebrates the spinal roots and ganglia develop independently of 
the spinal cord, and only subsequently become united with it. 
I have already insisted that this cannot be the primary condition ; 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 f. 
* [May 18, 1876. — Since the above was written Hensen has succeeded in showing that in mammals the 
rudiments of the posterior roots arise in a manner closely resembling that described in the present paper ; and 
I have myself, within the last few days, made observations which incline me to believe that the same holds 
good for the chick. My observations are as yet very incomplete.] 
t [May 18, 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.] 
