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.
Footnote 60: 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.
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.
Figs. B I, B II, B III. Three sections of a _Pristiurus_-embryo. B I is through the heart, B II through the anterior part of the dorsal region, and B III through a point slightly behind this. Drawn with a camera. (Zeiss CC ocul. 2.)
In B III there is visible a slight proliferation of cells from the dorsal summit of the neural canal.
In B II this proliferation definitely constitutes two club-shaped masses of cells (_pr_), both attached to the dorsal summit of the neural canal. The masses are the rudiments of the posterior nerve-roots.
In B I the rudiments of the posterior roots are of considerable length.
_pr._ rudiment of posterior roots. _nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._ subnotochordal rod. _ao._ dorsal aorta. _so._ somatopleure. _sp._ splanchnopleure. _al._ alimentary canal. _ht._ heart.
Fig. C. Section from a _Pristiurus_-embryo, slightly older than B. Camera. (Zeiss CC ocul. 2.) The embryo from which this figure was taken was slightly distorted in the process of removal from the blastoderm.