The second step in this stage (Pl. 22, fig. F) is comparable, as far as the connective-tissue is concerned, with the section of Torpedo (Pl. 22, fig. D d). The notochord (the histological details of whose structure are not inserted in this figure) is rather more developed, and the segmental duct, as was the case with the corresponding Torpedo-embryo, has become hollow at its anterior extremity.
The embryo from which the section was taken possessed five visceral clefts, but no trace of external gills.
In the section represented, though from a posterior part of the body, the dorsal nerve-rudiments have become considerably larger than in the last embryo; they now extend beyond the base of the neural canal. They are surrounded to a great extent by mesoblastic tissue, which, as in the case of the Torpedo, takes its origin from two sources, (1) from the commencing vertebral bodies, (2) from the summits of the muscle-plates.
It is in many cases very difficult, especially with chromic-acid specimens, to determine with certainty the limits of the rudiments of the posterior root.
In the best specimens a distinct bordering line can be seen, and it is, as a rule, possible to state the characters by which the cells of the nerve-rudiments and vertebral bodies differ. The more important of these are the following:—(1) The cells of the nerve-rudiment are distinctly smaller than those of the vertebral rudiment; (2) the cells of the nerve-rudiment are elongated, and have their long axis arranged parallel to the long axis of the nerve-rudiment, while the cells surrounding them are much more nearly circular.
The cells of the nerve-rudiment measure about 1/1600 × 1/4500 to 1/1600 × 1/3200 inch, those of the vertebral rudiment 1/1600 × 1/1900 inch. The greater difficulty experienced in distinguishing the nerve-rudiment from the connective-tissue in Pristiurus than in Torpedo arises from the fact that the connective-tissue is much looser and less condensed in the latter than in the former.
The connective-tissue cells which have grown out from the muscle-plates form a continuous arch over the dorsal surface of the neural tube (vide Pl. 22, fig. F): and in some specimens it is difficult to see whether the arch is formed by the rudiment of the posterior root or by connective-tissue. It is, however, quite easy with the best specimens to satisfy one's self that it is from the connective-tissue, and not the nerve-rudiment, that the dorsal investment of the neural canal is derived.
As in the previous case, the upper ends of each pair of posterior nerve-rudiments are quite separate from one another, and appear in sections to be united by a very narrow root to the walls of the neural canal at the position indicated in fig. F[52].
The cells forming the nerve-rudiments have undergone slight modifications; they are for the most part more distinctly elongated than in the earlier stage, and appear slightly smaller in comparison with the cells of the neural canal.
They possess as yet no distinctive characters of nerve-cells. They stain more deeply with osmic acid than the cells around them, but with hæmatoxylin there is but a very slight difference in intensity between their colouring and that of the neighbouring connective-tissue cells.