The attachment to the dorsal summit is not permanent, but before describing the further fate of the nerve-rudiments it is necessary to say a few words as to the neural crest. At the period when the nerves have begun to shift their attachment to the spinal cord, there makes its appearance, in Elasmobranchii, a longitudinal commissure connecting the dorsal ends of all the spinal nerves ([figs. 265], [266] com), as well as those of the vagus and glossopharyngeal nerves. This commissure has as yet only been found in a complete form in Elasmobranchii; but it is nevertheless to be regarded as a very important morphological structure.
Fig. 266. Spinal Nerves of Scyllium in longitudinal section to shew the commissure connecting them.
A. Section through a series of nerves.
B. Highly magnified view of the dorsal part of a single nerve, and of the commissure connected with it.
com. commissure; sp.g. ganglion of posterior root; ar. anterior root.
It is probable, though the point has not yet been definitely made out, that this commissure is derived from the neural crest, which appears therefore to separate into two cords, one connected with each set of dorsal roots.
Fig. 267. Section through the dorsal part of the trunk of a Torpedo embryo.
pr. posterior root of spinal nerve; g. spinal ganglion; n. nerve; ar. anterior root of spinal nerve; ch. notochord; nc. neural canal; mp. muscle-plate.
Returning to the original attachment of the nerve-rudiments to the medullary wall, it has been already stated that this attachment is not permanent. It becomes, in fact, at about the time of the appearance of the above commissure, either extremely delicate or absolutely interrupted.
The nerve-rudiment now becomes divided into three parts ([figs. 267] and [268]), (1) a proximal rounded portion, to which is attached the longitudinal commissure (pr); (2) an enlarged portion, forming the rudiment of a ganglion (g and sp g); (3) a distal portion, forming the commencement of the nerve (n). The proximal portion may very soon be observed to be united with the side of the spinal cord at a very considerable distance from its original point of attachment. Moreover the proximal portion of the nerve is attached, not by its extremity, but by its side, to the spinal cord ([fig. 268] x). The dorsal extremities of the posterior roots are therefore free.