188 
ME. F. M. BALFOUE ON THE DEVELOPMENT OF 
In a more ventral plane (fig. K 11) this relation is altered, and the posterior roots lie 
opposite the hinder parts of the muscle-plates. 
The nerves themselves are invested by the hyaline membrane spoken of above ; and 
surrounding this again there is present a delicate mesoblastic investment of spindle- 
shaped cells. 
Longitudinal sections also throw light upon the constitution of the anterior nerve-roots 
( vide fig. K 11, ar). In the two segments on the left-hand side in this figure the anterior 
roots are cut through as they are proceeding, in a more or less horizontal course, from 
the spinal cord to the muscle-plates. 
Where the section (which is not quite horizontal) passes through the plane of the 
notochord, as on the right-hand side, the anterior roots are cut transversely. Each root, 
in fact, changes its direction, and takes a downward course. 
The anterior roots are situated nearly opposite the middle of the muscle-plates: 
their section is much smaller than that of the posterior roots, and with hsematoxylin 
they stain more deeply than any of the other cells in the preparation. 
The anterior roots, so far as I have been able to observe, do not at this stage unite 
with the posterior ; but on this point I do not speak with any confidence. 
The period now arrived at forms a convenient break in the development of the spinal 
nerves; and I hope to treat the remainder of the subject, especially the changes in the 
ganglion, the development of the ganglion-cells, and of the nerve-fibres, in a subsequent 
paper. 
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 shown in 
Plate 18 . 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 show 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 11, and B ill. 
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 11), still in the region of the ear, has no trace of a nerve, and thus 
serves to show the early discontinuity of the posterior nerve-rudiments which arise from 
the brain. 
