Hamitton, Dzvzszon of Cells in Nervous System. 305 
of the central nervous system of the white rat differ from those 
arrived at by the observers quoted above : 
Ist. The ventricular mitoses predominate only in the 
earlier stages of development, while the nerve fiber tracts are 
still very narrow, the layer of germinating cells wide and the 
nerve cells immature. Even at this stage, numerous extra-ven- 
tricular mitoses are found. 
2nd. As development proceeds, there isa relative increase 
of extra-ventricular mitoses, so that by the end of the first day 
‘after birth, they are greatly in the majority. After this and up 
to the end of the 4th day, there is a slight proportional increase 
of the ventricular mitoses again. 
3rd. Cell division does not diminish steadily, but reaches 
a low point at the end of the Ist day and then rises again. This 
corresponds to the general suspension of growth in the whole 
animal just after birth and the gradual increase after that. The 
cells continue to multiply for at least four days after the stage 
described by Merk, and at the end of the fourth day the mi- 
toses are more numerous than at the end of the first. 
4th. The extra-ventricular mitoses are found chiefly in 
the gray matter, especially in the anterior horns, ns a few 
are in the fiber tracts. 
Part II. Nature oF THE DIVIDING CELLs. 
In the course of the determination of the number and per- 
sistence of mitoses in the central nervous system of the white 
rat, certain peculiarities in these dividing cells were noticed 
which led to a closer study of the nature of these cells found in 
the different localities. A brief examination of the cord at 
birth and for four days after birth is enough to convince one 
that the dividing cells are not all alike, that on the contrary, 
they differ in size and shape, in the character of the cytoplasm 
and in the thickness of the chromosomes. The variations in 
size are not so marked in the dividing cells of the brain as in 
the cord, but they are appreciable, and the other differences 
are as clearly marked as in the cord. In the brain and cord of 
the foetus the majority of the dividing cells are of one kind 
