234 C..M. JACKSON 
The middle zone (zona fasciculata) cells during the first week 
average about 9 » in diameter, being of approximately the same 
size throughout the zone. In the outer portion of this zone they 
increase to about 9.6 » at two and three weeks, increasing to 14 or 
15 » from ten weeks on. The cells in the inner portion of the 
middle zone appear slightly larger (10.3-10.7 u) at two and three 
weeks, but lag behind thereafter, reaching about 12 » in diameter 
from ten weeks on. . 
The inner zone (zona reticularis) cells during the first three 
weeks are similar in size to those in the adjacent outer part of 
the middle zone. Later, however, they lag behind, and at ten 
weeks are about equal in average diameter to those of the outer 
zone (8-10 u). In the adult rat, they are in average size the 
smallest cells of the suprarenal cortex. They do not vary greatly 
from 9. in average diameter throughout postnatal life. The 
small size of the cells is associated with the atrophic condition of 
the inner zone, which undergoes continual absorption (as will 
discussed later). 
The cells of the suprarenal medulla increase steadily from an 
average diameter of about 8 » at birth to 10 » or 11 » during the 
second and third weeks. At ten weeks, they average about 13 yu, 
and in the adult reach an average diameter of nearly 16 uz. 
The nuclear growth of the suprarenal parenchyma cells is in 
general less rapid than that of the cytoplasm, so that with the in- 
creasing volume of the cell the nucleus decreases in relative size, 
forming a steadily decreasing percentage of the average cell vol- 
ume (table 4). The outer zone nuclei apparently decrease in 
average diameter from 5.4 » at birth to 4.6 » at one week. They 
increase to about 5.5 4 at three weeks, thereafter remaining 
nearly constant in absolute size. In relative volume, however, 
they decrease from 44 per cent of the average cell volume in the 
new-born to about 23 per cent in the adult rat. 
The middle-zone nuclei likewise appear to decrease slightly dur- 
ing the first two weeks (from average diameter of 6.2 u to 5.6 y), 
but increase to 6.2 uw at three weeks. Thereafter the nuclei of 
the inner part of the zone (like the corresponding cells) increase 
but little in average diameter, while those of the outer part of 
