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NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 225 
to determine that other processes, instead of leaving the medullary wall, 
extend posteriorly in the wall and parallel with it to form the ventral 
fibre tract. The nerve process (Figure G) shows a differentiation in its 
distal portion into two deeply staining fibrils surrounded by more faintly 
staining plasma, the two fibrils dividing distally into three, which enter 
the fine processes with which the nerve ends. The nerve process on the 
other side of the same embryo (Figure H) does not, however, show this 
same evidence of histological differentiation. Here the nerve appears as 
a highly refractive fibril, and, while having a greater extent than that of 
its mate of the opposite side, is composed, except at its root, of a single 
undivided fibril. The connection of this fibril with the axis-cylinder 
process from a neuroblast cell in the ventral horn seems indisputable, 
since this passes directly through the limiting membrane at the base of 
the brain wall, and projects into the shrinkage space directly opposite 
the chief root of the nerve, as is shown at ax-eyl. I have no evidence 
to offer, such as that stated by His (’88, 789), for Mammals and other 
Vertebrates, of a migration of the neuroblasts from the “ inner layer” 
of medullary cells, nor do I find any evidence of migration of cells from 
the neural tube, as stated by Dohrn (91). I find at this stage neither 
nuclei connected with the roots of the nerve outside the neural tube, 
nor such as are half in and half out of the tube. 
The connection of the oculomotorius with cells of the mesocephalic 
ganglion is attained very quickly, and in embryos of 54 or 55 somites has 
already taken place. At this stage of development, as seen in embryos 
fixed with the corrosive sublimate-acetic mixture (Davidofl’s fluid), the 
nerve appears (Plate 8, Fig. 58) as a cellular strand, which extends from 
the inner side of the mesocephalic ganglion toward the ventral wall of 
the midbrain, with which the nerve unites by at least two main roots. 
To detect the proximal roots as well as the relations of these with 
medullary cells, sagittal sections are much more favorable than frontal, 
since the nerve roots are situated one behind the other.’ The fact that 
the nerve is several cells in thickness near the ganglion, while its calibre 
diminishes as it passes toward the brain wall, would naturally, if one 
were unacquainted with the conditions shown in the embryo of 52 
somites, lead to the inference that the growth of this nerve takes place 
from the ganglion toward the brain (vide Miss Platt, ’91, Mitrophanow, 
1 Also, for the reason already stated by His ('88”, p. 344) for spinal ventral 
nerves, that “die Wurzelbiindel treten in grósseren Abstiinden aus dem Riicken- 
mark hervor. Jedes Biindel bezieht seine Fasern aus einem entsprechend breiten 
Bezirk des Riickenmarks. Die Sammlung derselben erfolgt zum Theil noch inner- 
halb des Markes, zum Theil erst in der Leibeswand.” 
VOL, XXXI. — NO. 7. 6 
