240 Cc. M. JACKSON 
observed, excepting one of the nine cases at three weeks. The 
average number of mitoses again decreases somewhat slowly 
from 27 at 14 days to 24 at 21 days, 11 at 56 days, 4 at 67 to 
94 days, 2 at 112 to 138 days, and 1 at 340 days. In other 
adults no mitoses were observed in the suprarenal. 
The relative rate of mitosis (‘mitotic index’ of Minot) de- 
creases more rapidly than is indicated by the figures for the 
absolute frequency per section shown in table 5A. This is 
because the size of the gland, and consequently the total number 
of cells in a section, is progressively increasing. Thus between 
birth and three weeks of age the suprarenal gland of the albino 
rat has increased about four times in weight (and volume). 
This corresponds to an increase in diameter of about 1.6, or 
of about 2.6 times in the area of a cross-section. Since the 
size of the cells has not greatly increased during this time (table 
4), the number of cells in a corresponding section of the gland 
is probably twice as great at three weeks as in the new-born. 
Thus the relative rate of mitosis would be obtained by com- 
paring the number of mitoses in a section at birth (20) with 
half the number in a section at three weeks (half of 24 = 12). 
Therefore, although the absolute number of mitoses per section 
increases in the second and third weeks, it is doubtful whether 
the relative rate of mitosis exceeds that of the new-born. 
The distribution of the mitoses in the various zones of the 
cortex and the medulla is also shown in table 5A. The rate 
in the medulla is in general nearly parallel with that for the 
whole gland, decreasing from an average of 6 per section in the 
new-born to 3 at one week, and gradually increasing again to 
6 at fourteen days. Thereafter the number of mitoses decreases, 
and none are found in the medulla after eight weeks, though 
continuing to occur in the cortex. The later growth of the 
medulla must therefore be due entirely to the growth in the 
preéxisting cells, which continue to increase in average size 
(table 4). 
As to localization in the various regions of the cortex, the 
number of mitoses in the outer zone (glomerulosa) is strikingly 
large, although this zone is the narrowest of the three. When 
