THE OESTROUS CYCLE IN THE MOUSE 301 
2. MATERIAL AND METHOD 
Variation in the duration of the oestrous cycle has been reported 
in nearly all mammals studied, not only in closely allied forms, 
but within the limits of the same species. It seemed desirable, 
therefore, to include some individuals from each strain of mice 
in our colony. Several variations in other phenomena peculiar 
to certain strains had formerly been noticed, the most striking 
being the difference of reaction to ether anaesthesia: Albinos in 
our stock are very susceptible, while brown and black are quite 
resistant. It was thought important to find if there was any 
variation in the oestrous cycle typical of different strains. The 
stock chosen included brown, black, albino, dominant white 
with black eyes, gray, agouti, and yellow, as well as hybrids of 
these strains. More than ninety animals have been used in 
this work, the majority being young virgin mice. A few who had 
born litters were included for comparison, but in all cases records 
were not begun until the animals had been separated from males 
for at least a month after the removal of their young. 
It has long been known that to maintain the oestrous rhythm 
the animals must be healthy, well fed, and under uniform envi- 
ronmental conditions. To eliminate crowding, no more than four 
animals were placed in one cage. Cages had a floor space of 
850 sq. cm. and a capacity of 17,000 ce. 
A constant supply of water was accessible to the mice through 
small holes in the ends of test tube containers. The food con- 
sisted of oats, cracked corn, sunflower seed, and dog biscuits. 
Usually a little hay was added as nest-building material. 
During the winter the animals were kept in a heated room. 
Methods 
Stockard and Papanicolaou (’17) have shown the most reliable 
criterion of the condition of heat in the guinea-pig to be the cell 
content of the vaginal fluid. Their technique, slightly modified 
because of the smaller size of the mouse, was followed. Some 
animals were examined three times daily to get the exact time 
relations of the various phases of the cell changes, but for the 
