420 HAYATO ARAI 
a minimum at about twenty days. This striking phenomena is 
associated with the rapid increase of the larger ova during this 
period. From twenty days up to about thirty-six days, the 
percentage value increases again, though the absolute number 
of the small ova is decreased, owing to the appearance of the 
larger ova (table 2). From thirty-six to sixty-four days the 
percentage values for the number of small ova are approximately 
constant. This is associated with slight numerical changes in 
the number of ova of all sizes. 
2. Between 64 and 110 days, in those ovaries in which cor- 
pora lutea have not yet appeared, the percentage values of the 
small ova range from 94.6 per cent to 97 per cent. In the ovaries 
in which the corpora lutea have appeared, but excluding the preg- 
nant animals, the number of small ova ranges from 93.7 per cent 
to 96.8 per cent, showing no change in the number of these cells 
in relation to the appearance of the corpora lutea. 
3. From 140 to 947 days the corpora lutea are always present 
and the percentage values fluctuate about a mean value of 94 
per cent, the very oldest rat giving 89 per cent. Therefore, 
generally speaking, the percentage values of the small ova (less 
than 20 u diameters) remain constant in the older rats. 
4. In the pregnant rats—four cases—we find the percentage 
values 95 to 97 per cent in three cases and 88 per cent in one, 
which is the youngest. For this unusual value we have no ex- 
planation. 
From the foregoing it seems that the percentage values for the 
ova under 20 uw decrease rapidly from one day after birth till 
twenty days. ‘This is followed by an increase of 4 per cent up 
to thirty-six days. From thirty-six days on to the end of the 
series the percentage values remain nearly constant (the four — 
pregnant cases are not included in this general statement). 
The number of ova 20 to 40 uw in diameter show percentage 
values which increase rapidly from three days up to seven days, 
at which age they reach a maximum of 4.6 per cent. This ini- 
tial increase is followed by lower values up to sixty-four days, 
after which there is a slight tendency to higher percentages 
with advancing age. 
