THE SPINAL NERVES IN ELASMOBRANCH EISHES. 
183 
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, (l)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-rudi- 
ment 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 r b 3 ^ o X 4 i oo t° TFoo X 3 - 2 V 0 inch, 
those of the vertebral rudiment xtToo Xrvb'o 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 con- 
tinuous arch over the dorsal surface of the neural tube ( vide Plate 17. 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 speci- 
mens 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 *. 
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 heematoxylin there is but a very 
slight difference in intensity between their colouring and that of the neighbouring 
connective-tissue cells. 
MDCCCLXXVI. 
* The artist has not been very successful in rendering this figure. 
2 C 
