IN ELASMOBRANCH FISHES. 189 



first discontinuous, and the points from which they originally 

 spring remain as those by which they are permanently attached 

 to the spinal cord, and do not, as in the case of the posterior 

 roots, undergo a change of position. The anterior roots arise, 

 not vertically below, but opposite the intervals between the 

 posterior roots. 



The anterior roots are at first quite separate from the pos- 

 terior roots ; but soon after the differentiation of the posterior 

 rudiment into a root, ganglion, and nerve, a junction is effected 

 between each posterior nerve and the corresponding anterior 

 root. The junction is from the first at some little distance from 

 the ganglion. 



Investigators have hitherto described the spinal nerves as 

 formed from part of the mesoblast of the protovertebrae. His 

 alone, so far as I know, takes a different view. 



His's l observations lead him to the conclusion that the pos- 

 terior roots are developed as ingrowths from the external epiblast 

 into the space between the protovertebrae and the neural canal. 

 These subsequently become constricted off, unite with the neural 

 canal and form spinal nerves. 



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 2 . 



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 con- 

 siderable light is the primitive origin of nerves. 



So long as it was admitted that the spinal and cerebral nerves 



1 Erste Anlage des Wirbelthier-Leibes. 



2 [May 1 8, 1876. Since the above was written Hensen has succeeded in shewing 

 that in mammals the rudiments of the posterior roots arise in a manner closely re- 

 sembling 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.] 



