328 EDGAR ALLEN 
Stage P, as diagnosed by the smear method, may be less 
than one day, because leucocytes may not entirely disappear 
from the superficial vaginal epithelium until after growth in the 
deep layers is well under way. Stage O usually lasts one or two 
days, but in several cases unbroken O smears have continued 
for nine days, and four days of heat are not uncommon. M, 
and M, stages usually last a day each and show little variation. 
Curve of cycle length 
Number of cycles 
ie th ee Ll. 
Length of cycles in days 
Graph 1 
The data accumulated in this study indicate no correlation 
between the number of ova produced at an oestrus and the 
length of the cycle. 
While variability of cycle length is great in the same animal, 
it is still greater in different strains. Plotting the number of 
cycles against their duration in mice of different coat color gave 
the following curves. The mode of the curve for brown coat 
color is six days. This strain has the greatest number of unusu- 
ee See 
