MITOSIS IN CENTRAL NERVOUS SYSTEM 567 
been prepared in the same manner: 70-day—5 x 4, 5.5 x 5, 6x4, 
6 x 3 micra; 120 day—6 x 4,4x 5,4x4micra. None were found 
measuring more than 6 x 4 micra. 
The dividing cells which I have enumerated in this study are 
believed not to be leucocytes or lymphocytes for the following 
reasons: the size of the nuclei in leucocytes found in blood vessels 
in my preparations is from. 3.2 x 3.2 micra to 3.8 x 3.8 micra, 
while the chromatin material in the nerve nuclei measures from 
4 x 4.2 in anaphase to 7.5 x 8 micra in prophase and metaphase. 
Moreover, after the dividing cells have disappeared from all other 
areas they are still to be found in the limited zone along the lat- 
eral ventricles of the cerebrum which has already been described. 
They are to be found also in the neighborhood of capillaries but 
this association is not constant. 
The differentiation from lymphocytes is easier. These are 
found scattered through the tissue; they are smaller than the 
leucocytes and are readily recognized by the characteristic nuclei, 
their chromatin being gathered into one, two or more well sepa- 
rated, densely stained spherical masses, the outlines of which are 
always smooth and regular, lying in a clear nuclear matrix 
bounded by a limiting membrane of uniform thickness. 
¥ 
CONCLUSIONS 
1. Mitosis ceases in the central nervous system of the albino 
rat as follows: (a) No mitoses are found in the cord at any level 
after the eighteenth day. This is somewhat previous to the 
complete cellular differentiation of the wall of the central canal, 
a stage accomplished in the thoracic region by the twentieth day 
after birth, in the lumbar region by the thirtieth day after birth, 
while at this last-named age the cervical level shows in occasional 
sections a portion of the wall still incomplete. (b) In the cerebel- 
lum mitosis ceases when the migration of the cells in the external 
granule layer is complete, a condition reached between the twen- 
tieth and twenty-fifth day after birth. (c) In the cerebrum mitosis 
continues with a considerable degree of activity to the twentieth 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL, 22, NO. 6 
