This resemblance would even extend beyond mere external form. Let the ventral nervous cord of the common earthworm, Lumbricus agricola, be used for comparison[58], a transverse section of which is represented by Leydig[59] and Claparède. In this we find that on the ventral surface (the Annelidan ventral surface) of the nervous cord the ganglion-cells (grey matter) (k) are situated, and on the dorsal side the nerve-fibres or 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 Elasmobranchii 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 shew the à priori possibility of the formation of a spinal canal and the à 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[60].
Plate 22.
Fig. A. Section through the dorsal region of an embryo of Scyllium stellare, 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.
nc. neural canal. mp. muscle-plate. ch. notochord. x. subnotochordal rod. ao. rudiment of dorsal aorta. so. somatopleure. sp. splanchnopleure. al. alimentary tract. All the parts of the section except the spinal cord are drawn somewhat diagrammatically.