THE OESTROUS CYCLE IN THE MOUSE 329 
ally long cycles. That for black and albino is four days. AI- 
binos in our stock are browns minus the color factor. The mode 
for yellow and gray falls at five days. The identity of yellow 
and gray is more significant, since these grays are recessive deriva- 
tives of the yellow strain. The stock used included a black- 
eyed, dominant white mouse which has a lethal factor. Her 
Variations in cycle length in different strains 
—— Yellow 
Fas CUReY, 
Number of cycles 
Length of cycles in days 
Graph 2 
average for seven cycles was 7.7 days, which, compared to the 
mode of four days for the albino strain, is a striking difference. 
This is only one case, but it raises the question of the relation 
of this lethal factor to the duration of the oestrous cycle. These 
findings indicate that a genetic factor is accountable for some 
of the variation in cycle length among different strains of mice. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 30, No. 3 
