I will only add that, not long after the stage last described, the posterior root unites with the anterior root at a considerable distance below the cord: this is shewn in Pl. 23, fig. L. Still later the portion of the root between the ganglion and the spinal cord becomes converted into nerve-fibres, and the ganglion becomes still further removed from the cord, while at the same time it appears distinctly divided into two parts.
As regards the development of the cranial nerves, I have made a few observations, which, though confessedly incomplete, I would desire to mention here, because, imperfect as they are, they seem to shew that in Elasmobranch Fishes the cranial nerves resemble the spinal nerves in arising as outgrowths from the central nervous system.
I have given a figure of the development of a posterior root of a cranial nerve in fig. M I. The section is taken from the same embryo as figs. B I, B II, and B III.
It passes through the anterior portion of a thickening of the external epiblast, which eventually becomes involuted as the auditory vesicle.
The posterior root of a nerve (VII) is seen growing out from the summit of the hind brain in precisely the same manner that the posterior roots of the spinal nerves grow out from the spinal cord: it is the rudiment of the seventh or facial nerve. The section behind this (fig. M II), still in the region of the ear, has no trace of a nerve, and thus serves to shew the early discontinuity of the posterior nerve-rudiments which arise from the brain.
I have as yet failed to detect any cranial anterior roots like those of the spinal nerves[53]. The similarity in development between the cranial and spinal nerves is especially interesting, as forming an important addition to the evidence which at present exists that the cranial nerves are only to be looked on as spinal nerves, especially modified in connexion with the changes which the anterior extremity of the body has undergone in existing vertebrates.
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My results may be summarized as follows:—
Along the extreme dorsal summit of the spinal cord there arises on each side a continuous outgrowth.
From each outgrowth processes corresponding in number to the muscle-plates grow downwards. These are the posterior nerve-rudiments.