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Among these migrating cells I also recognize, as did they, the indifferent cells 
of Schaper. In several sections, however, cells with large rounded or elongated 
nuclei and cytoplasm drawm out to a point at one side were observed among 
the fibers of the ventral nerve-root, migrating with the indifferent cells. In 
these cells I recognize the neurohlasts of Schaper.. They are much fewer in 
number than the indifferent cells but are distributed indiscriminately among 
them. When observed in the spinal nerve-roots the tapering end is usually 
directed peripherally. This, also, is in accordance with the usual position 
of the neurohlasts in the mantle layer. The orientation of the cells in 
the neural tube is such that two general courses of migration into the ven- 
tral nerve-root may he recognized; the one directly outward from the ventral 
zone; the other ventro-laterally toward the base of the nerve-root, from the 
region in which later the lateral horn of the gray matter arises. 
In transverse sections of embryos of the pig 6 and 7 mm. long, breaches 
in the external limiting membrane of the neural tube occur quite frequently 
in the region of the dorsal nerve-root. Through these breaches medullary 
cells migrate so freely that in many sections lines of cells practically touching 
each other end to end can be traced from the mantle layer Into the proximal 
part of the dorsal nerve-root (fig. II). Further evidence for the migration of 
medullary cells into the dorsal nerve-roots is seen in the fact that in many 
sections of embryos 6 and 7 mm. long, where no breaches occur, cells are 
found in contact with the external limiting membrane in the region of the 
dorsal nerve-root. In embryos 9 mm. and over in length this area, as shown 
in fig. I, is always occupied by fibers of the dorsal nerve-root, and rarely are 
cells found among them. Two general courses of migration may also be rec- 
ognized in the dorsal region. One of these courses has its origin in the dorsal 
zone. Some of its cells move in a slight curve directly toward the dorsal 
nerve-root, others pass from the most dorsal region along the inner surface of 
the external limiting membrane. The other course tends dorso-laterally from 
regions ventral to the dorsal nerve-root. The cells of the latter course probably 
originate in the same region as those which move ventro-laterally toward the 
ventral nerve-root. 
Among the cells migrating into the dorsal nerve-roots, I recognize both the 
“indifferent cells” and the “neurohlasts” of Schaper. 
My observations do not permit me to conclude how early the first migration 
of cells from the neural tube takes place. In studying a very young embryo 
in which, however, the external limiting membrane was completely formed, no 
migrating cells could be observed. It is probable, therefore, that cells do not 
migrate out of the neural tube until the fibers of the nerve-roots have pene- 
trated the external limiting membrane. 
As observed above, cells are rarely found among the fibers of the dorsal 
nerve-root near the external limiting membrane inside the neural tube, in 
embryos 9 mm. and over in length. Migration into the dorsal nerve-root pro- 
bably ceases before a length of 9 mm. is reached. Migration into the ventral 
nerve-root continues longer and seems to be most active in embryos from 10 to 
13 mm. in length. In embryos over 13 mm. in length migration is rarely 
observed and probably does not continue far beyond that stage. 
There are four points to which the writer wishes to call special attention 
in this paper. (1) Medullary cells migrate into both dorsal and ventral nerve- 
roots. (2) These migrating cells seem to have their origin in more or less 
