458 HAYATO ARAI 
4. In general the number of ova decreases with age. The 
total number of ova in both ovaries decreases rapidly from 35,100 
at birth to about 11,000 at twenty-three days. From twenty- 
three to sixty-three days the number is nearly constant (11,000 
to 10,000). It then decreases again rapidly (to about 6600) at 
seventy days. During this last period ovulation usually occurs. 
From seventy days up to the thirty-one months there is a slow 
decrease to about 2000 ova. In general, this decrease results 
mainly from the degeneration of the primitive ova, but in part 
from that of the definitive ova (tables 2 and 3, charts 1 and 2). 
5. In non-pregnant rats the percentage of larger-sized ova to 
the total number remains nearly constant. In young pregnant 
rats the percentage of ova more than 20 u in diameter is greater 
than in the non-pregnant rat, but in older pregnant rats this per- 
centage decreases. These conclusions are; however, based on 
four instances only. 
6. The graphs illustrating the change in the total number of 
ova are similar in form whether they are based on the body 
weight or the body length (table 6, chart 3). 
7. With the increasing weight of the ovary the total number of 
ova decreases. The increase in weight is associated before pu- 
berty principally with the formation of large degenerate follicles 
together with mature follicles and growth of connective tissue, 
but after puberty the increase depends mainly on the accumu- 
lation of small corpora lutea in addition to the mature and degen- 
erate follicles and connective tissue. After puberty the number of 
large corpora lutea is about the same in all ovaries, and these, 
therefore, are not responsible for the regular age changes in the 
weight of ovaries (table 10, chart 4). 
8. In albino rats the new formation of the egg cells takes place 
after birth from the germinal epithelium. ‘These ova grow in 
situ and, as development proceeds, are covered by the adjacent 
epithelial cells and extend into the stroma, passing through the 
tunica albuginea. From these newly formed germ cells the defini- 
tive ova appear to develop, beginning from about the second 
week after birth, and the formation of them is most active in the 
period between the third and ninth weeks. During puberty 
