B52 EZRA ALLEN 
As a basis for determining the comparative rate of cell divi- 
sion the number of mitoses per cubic millimeter was obtained. 
With the aid of the camera lucida or projection apparatus, draw- 
ings were made of the first and last sections of a series. These 
drawings were then measured by the planimeter and the mean 
area taken. Since the sections were of a fixed thickness (8 micra) 
the volume of the tissue in question was readily calculated. The 
number of dividing cells in the series of sections had already been 
determined, so that the necessary data for finding the number of 
dividing cells per cubic millimeter of tissue were at hand. 
STATEMENT OF RESULTS 
The distribution of mitoses 
A brief statement will first be made under this head to be fol- 
lowed by more detailed explanation. The results of this study 
show that at birth mitoses are occurring in each division of the 
central nervous system, at all levels of the cord, cerebellum and 
cerebrum. An examination of figures 1 to 4 will make it evident 
that the general distribution, however, is unequal. Table 2 
(p. 557) indicates at different ages the relative mitotic activity 
per cubic millimeter at the three principal levels of the cord and 
at one level of the cerebellum and cerebrum respectively. Figures 
2 to 4 show the relative longitudinal distribution at three levels 
in the cerebrum, as well as the fact that in this organ the region 
of greatest activity is in the walls of the lateral ventricles. In 
the cerebellum dividing cells are most abundant in the cortex. 
Table 2 shows the relative rate of mitosis at different levels for 
the ages from one day to twenty-five days after birth. This 
table shows also that cell division stops earliest in the cord, a 
little later in the cerebellum and still later in the cerebrum. 
With these data in mind we may pass to the details. 
1. The distribution as it appears in frontal section will be better 
appreciated by a preliminary consideration of the embryonic 
differentiations of tissue as they appear in the central nervous 
system. The original epiblastic layer forming the wall of the 
neural tube is early converted into a protoplasmic framework 
