NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 231 
parallel to the neural tube, toward van Wijhe’s third somite. In later 
stages the nerve divides into two branches, one passing along the inner 
side of the somite, and the other along the outer side, At first the nerve 
root, which appears as a plasma outflow from the neural tube, is of the 
thickness of one, or at most two, medullary cells. Later the roots 
increase in thickness, apparently by the continued outgrowth of plasma 
from the neural tube, as well as by the migration of cells from the 
ventral horn of the medulla. The larger size and different staining 
qualities of the medullary cells enabled him to distinguish them from 
the mesodermal cells in their vicinity. Such (medullary) cells are often 
found with a part of the nucleus within and a part without the medul- 
lary wall, This outflow (migration) of medullary cells takes place also 
in later stages after the white substance has become quite thick on the 
side wall of the neural tube.! 
My observations upon the development of the abducens differ from 
those of Dohrn, as in the case of the oculomotorius, inasmuch us I find 
the nerve to arise from axis cylinder processes of neuroblasts in the 
ventral horn of the medulla, and therefore to resemble in its mode of 
development that of a ventral spinal nerve, as stated by His (89). At 
the earliest stage which I have been able to detect the abducens, it 
possesses but a single root, formed by the processes of several neuro- 
blasts, as is represented in Figure J. The union of these takes place 
just outside the medullary wall, yet peripherally the nerve appears 
as a single process with deeply staining axis and a more lightly stained 
sheath. I find neither at this stage nor in later'stages any convincing 
evidence of a migration of the neuroblast cells from the wall of the 
neural tube. In later stages of development sections show that the 
nuclei seen along the course of the nerve are distinctly peripheral in re- 
lation to its fibres. Even the phenomena presented in sections of 
embryos fixed with corrosive-sublimate acetic, such as are represented in 
Figures 62-65 (Plate 9), warrant in my judgment only the inference 
that the nuclei of the nerve are peripheral, as held by Miss Platt COL): 
The darker appearance of the nuclei lying upon the nerve results more 
from the opaqueness of the nerve than from any peculiar staining proper- 
ties of the nuclei. During development the number of roots in the 
nerve increases from one to three or four, the number being variable 
even upon the two sides of the same embryo. The method of develop- 
1 Since Marshall (’81), van Wijhe (’82), and Miss Platt (91) never saw the early 
stages of development of the abducens, it is unnecessary to restate, their results in 
this connection. 
