416 HAYATO ARAI 
days. At this age the corpora lutea may appear, that is at about 
sixty-four days ovulation occurs. From sixty-four days on the 
total number of ova decreases slowly but steadily to about 2,000 
at 947 days. This was the oldest rat available for study. Using 
the ratio of the span of life, 1 to 30, which we commonly employ 
when comparing the rat with man, 947 days for the rat is equiva- 
lent to seventy-eight years for man. 
Ova of larger sizes 
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Age—days 
Chart 2. Showing the total number of ova, as well as the number of ova of 
different sizes in the albino rat at different ages (condensed). 
On account of the high individual variation, I have averaged 
the data in table 2 and given the averages in table 3 and chart 2 
with the hope that the data thus arranged might reveal more 
clearly the real tendency to the changes in the number of ova 
according to the age of the rat. 
From birth up to 110 days the number of ova in the ovaries 
of the rats were grouped together within ten-day intervals, and 
after 110 days at intervals of about fifty days. The results of 
this grouping are shown in table 3 and chart 2. We notice that 
from the averages of four to twenty-three days the mean total 
number decreases very rapidly, from 27,483 to 10,746 ova, and 
