44 
GEORGE E. NICHOLES. 
of the neural crests. These neural crests are known to 
extend forward in development to the front end of the 
mesencephalon, being interrupted, however, in the region of 
the ear. Tlie extent of the neural crests, therefore, coincides 
strictly with the regions in which occur the giant-cells of the 
spinal cord and the cells of the “Dachkern.” 
Thus, in their dorsal position adjacent to the cavity of the 
neural tube, in their general appearance, in their great size, 
in their' possession of one particularly conspicuous non- 
medullated fibre, in their peculiar staining reactions and in 
their tendency to atrophy at the end of embryonic life, the 
cells of the “ Dachkern” show a striking resemblance to the 
giant-cells of the spinal cord. 
I have ali'eady called attention (’09, ’12) to the remark- 
able elasticity of Reissner’s fibre and to its behaviour in recoil 
in a manner quite unknown among nerve-fibres. Horsley has 
pointed out that in its failure to show degenerative changes 
after section it appears quite unlike a nerve-fibi’e, and this 
the results of rny own experiments, some of which are 
already lecorded (’12), entirely confirm. In its staining 
reactions, too, the fibre is altogether distinctive. 
After continued investigation into the structure and mode 
of occurrence of Reissner’s fibre in all classes of vertebrates, 
I find myself almost completely at variance with the views 
expressed by Sargent, and I repeat here the assertion that 
the fibre is not a nerve-tract. It arises fi’om the specialised 
epithelium of the sub-commissural organ, which has markedly 
the character of a sensory epithelium. It is first discernible 
at a point far forward beneath (anterior to) the posterior 
commissure, where it is formed by the coalescence of numerous 
fine fibrillae, resembling long cilia, from the elongated epen- 
dymal cells ; these fibrillae continue to join it along its whole 
course beneath the posterior commissure (and probably also 
in the canalis centralis) ; but it never arises, either 
wholly or in part, from any point in the brain dorsal 
(posterior) to the posterior commissure iii any form which I 
have studied, except, perhaps, in a comparatively few cases 
