192 
ME. E. M. BALEOTJE ON THE DEVELOPMENT OE 
white matter ( h ). If the folding that I have supposed were to take place, the grey and 
white matters would have very nearly the relative situations which they have in the 
Vertebrate spinal cord. 
The grey matter would be situated in the interior and surround the epithelium of 
the central canal, and the white matter would nearly surround the grey and form the 
anterior white commissure. The nerves would then arise, not from the sides of the 
nervous cord as in existing Vertebrates, but from its extreme ventral summit. 
One of the most striking features which I have brought to light with reference to the 
development of the posterior roots, is the fact of their growing out from the extreme 
dorsal summit of the neural canal — a position analogous to the ventral summit of the 
Annelidan nervous cord. Thus the posterior roots of the nerves in Elasmobranchs arise 
in the exact manner which might have been anticipated were the spinal cord due to 
such a folding as I have suggested. The argument from the nerves becomes the 
stronger, from the great peculiarity in the position of the outgrowth, a feature which 
would be most perplexing without some such explanation as I have proposed. The 
central epithelium of the neural canal according to this view represents the external 
skin ; and its ciliation is to be explained as a remnant of the ciliation of the external 
skin now found amongst many of the lower Annelids. 
I have, however, employed the comparison of the Vertebrate and Annelidan nervous 
cords, not so much to prove a genetic relation between the two as to show the a, 'priori 
possibility of the formation of a spinal canal and the a posteriori evidence we have of 
the Vertebrate spinal canal having been formed in the way indicated. 
I have not made use of what is really the strongest argument for my view, viz. that 
the embryonic mode of formation of the spinal canal, by a folding in of the external 
epiblast, is the very method by which I have supposed the spinal canal to have been 
formed in the ancestors of Vertebrates. 
My object has been to suggest a meaning for the peculiar primitive position of the 
posterior roots, rather than to attempt to explain in full the origin of the spinal canal. 
Explanation of the Plates*'. 
PLATE 16. 
Pig. A. Section through the dorsal region of an embryo of Scyllium stellar e, with the 
rudiments of two visceral clefts. The section illustrates the general features 
at a period anterior to the appearance of the posterior nerve-roots. 
no , neural canal; mp, muscle-plate; cA, notochord; x, subnoto chordal 
* The figures on these Plates give a fair general idea of the appearance presented by the developing spinal 
nerves ; but the finer details of the original drawings have in several cases become lost in the process of copying. 
The figures which are tinted represent sections of embryos hardened in osmic acid ; those without colour 
sections of embryos hardened in chromic acid. 
