IOWA ACADEMY OF SCIENCE 
219 
definite regions in the neural tube. (3) Migration from the neural tube seems 
to be limited to a comparatively short period during embryonic development. 
(4) While the majority of the cells which migrate into the dorsal and ventral 
nerve-roots are cells of an indifferent character, cells may be recognized among 
them, which are to be regarded as the neurohlasts of Schaper. 
These observations give rise to an important question. What is the destiny 
of those cells which migrate from the neural tube into the dorsal and ventral 
nerve roots? This question the writer hopes to consider more fully in a later 
paper. But, as the way has been opened for a priori speculation a few sug- 
gestions may be ventured here. 
Harrison (’01) has already suggested the possibility that the medullary 
cells which he saw migrating into the ventral nerve-roots of embryos of the 
salmon may wander farther peripherally, i. e., into the sympathetic ganglia, 
and there give rise to sympathetic motor neurones. Bardeen (’03) suggests 
that the cells which wander out from the spinal ganglia and cord along with 
the bundles of axis-cylinder processes of the nerve-roots may take some part in 
the formation of the neurilemma, but believes with Vignal and Gurwitsch that 
in mammals the cells composing the neurilemma arise largely from mesen- 
chyme. Harrison’s experimental observations on the tadpole (’06) seem to 
prove that some of the elongated cells found among the fibers of developing 
nerves, which become cells of the neurilemma, are derived from spinal ganglia. 
Carpenter and Main (’07) “feel sure” that some of the medullary cells which 
escape from the neural tube become cells of the neurilemma, and there sub- 
serve a supporting function similar to that of the neuroglia cells in the central 
nervous system. 
Thus far only cells of an indifferent character have been considered. May 
not the fact that neuroblasts migrate with the indifferent cells into both dorsal 
and ventral nerve-roots shed some new light on the entire problem? To trace 
these cells to their ultimate destination is a matter of extreme difficulty, if not 
impossibility, with the aid of present staining methods alone. In the light 
of Schaper’s researches, however, we must conclude that the neurohlasts 
which migrate into the dorsal and ventral nerve-roots have already undergone 
differentation and must develop into neurones. Having once passed beyond 
the borders of the neural tube they must migrate farther peripherally. Nothing 
seems more probable, therefore, than that they should wander along the visceral 
ramus and give rise to neurones in the sympathetic ganglia. Any conclusion, 
however, on this point would at present be hasty. We feel that these migrant 
cells, both the “Indifferent cells” and the “neurohlasts” of Schaper, are fraught 
with great potentialities and invite the most careful scrutiny into their fate. 
LITERATURE CITED. 
Bardeen, C. R., ’03, The Growth and Histogenesis of the Cerebro-spinal Nerves in 
Mammals. Amer. Journ. Anat., Vol. 2, No. 2, p. 231. 
Carpenter, F. W., ’06, The development of the Oculomotor Nerve, the ciliary Ganglion, 
and the Abducent Nerve, in the Chick, Bull. Mus. Comp. Zool. Harvard Coll., Vol. 48, 
No. 2, p. 241. 
Carpenter, P, W. and Main, R. C., ’07, The Migration of Medullary Cells into the Ven- 
tral Nerve-roots of Pig Embryos, Anatomischer Anzieger, Vol. 31, No. 11-12, p. 303. 
Gurwitsch, A., ’00, Die Histogenese der Schwann’ schen Sheide, Arch. f. Anat. u. Physio., 
Jahrg. 1900, Anat, Abt., Heft 1-2, p. 85. 
