Considering the clearness of this formation in this embryo, as well as in the embryo belonging to the stage under description, there cannot be much doubt that at the first formation of the posterior rudiments a continuous outgrowth arises from the spinal cord, and that only at a later period do the junctions of the roots with the cord become separated and distinct for each nerve.
I now return to the more complete series of Pristiurus-embryos, the development of whose spinal nerves I have been able to observe.
Second Stage of the Spinal Nerves in Pristiurus.
In the youngest of these (Pl. 22, fig. E) the notochord has undergone but very slight changes, but the segmental duct has made its appearance, and is as much developed as in the Torpedo-embryo from which fig. D b was taken.
(The embryo from which fig. E a was derived had three visceral clefts.)
There have not as yet appeared any connective-tissue cells dorsal to the top of the muscle-plates, so that the posterior nerve-rudiments are still quite free and distinct.
The cells composing them are smaller than the cells of the neural canal; they are round and nucleated; and, indeed, in their histological constitution the nerve-rudiments exhibit no important deviations from the previous stage, and they have hardly increased in size. In their mode of attachment to the neural tube an important change has, however, already commenced to be visible.
In the previous stage the two nerve-rudiments met above the summit of the spinal cord and were broadly attached to it there; now their points of attachment have glided a short distance down the sides of the spinal cord[51].
The two nerve-rudiments have therefore ceased to meet above the summit of the canal; and in addition to this they appear in section to narrow very much before becoming united with its walls, so that their junctions with these appear in a transverse section to be effected by at most one or two cells, and are, comparatively speaking, very difficult to observe.
In an embryo but slightly older than that represented in Fig. E a the first rudiment of the anterior root becomes visible. This appears, precisely as in Torpedo, in the form of a small projection from the ventral corner of the spinal cord (fig. E b, ar).