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Hamitton, Dzviszon of Cells in Nervous System. ~- 309 
form fully half the width of the section, the ependyma being 
reduced to a thin double layer. Within these tracts are found 
in the cord neuroglia cells of different sizes and, in the lateral 
tracts especially, scattered neuroblasts and multipolar cells. In 
the fiber tracts of the foetal brain there are, besides the neu- 
roglia cells, many neuroblasts which are found lying diagonally 
and horizontally and vertically. Evidently the neuroblasts 
which are formed in the ependyma make a half revolution in 
this layer on their way to the cortical layer. Later stages show 
fewer neuroblasts, in the fiber tracts and finally—in the 4 days 
specimen—only neuroglia cells. The dividing cells in the white 
tracts are not very numerous at any stage of development, and 
when. present they are usually of the same variety, without 
cytoplasm. Large ones are sometimes found in the anterior 
and lateral tracts of the cord, near the gray matter, but usually 
the mitotic figures are small and delicate. 
It is evident that in the later stages of development the 
increase of cells.in the central nervous system of the white rat 
does not take place through the division of one kind of cell 
only, that there is not an indifferent dividing cell, but several 
varieties of cells which may undergo division. 
Referring to the literature on this subject one naturally 
begins with the classical work of His (2) on the central nervous 
system of the human embryo of one month. 
According to His the original epithelial elements which 
line the neural tube become differentiated into two kinds of 
cells, only one of which is capable of multiplication. These 
are the germinal cells which appear among the epithelial as 
round or oval bodies measuring in man 10 X 14 4. The nucleus 
is usually in one of the stages of mitosis, but when resting has 
a thick nuclear membrane and scattered chromatin masses. 
These germinal cells divide in the layers of ependyma nearest 
the ventricle, and their offspring migrate from this region out 
toward the periphery, grouping themselves together to form the 
layer of cells which His called the mantle layer. During their 
passage, or after completing it, they undergo changes which 
result in the formation of neuroblasts, that is, the oval cell be- 
