298 EDGAR ALLEN 
slight.”’ In many animals I have found them to be either entirely 
absent or so slight as to make an accurate diagnosis impossible. 
Discussing the sexual phenomena of rodents, Marshall says: 
‘‘Mus decumanus and M. musculus are known to experience a 
recurrence of the dioestrous cycle for more than nine months of 
the year in the absence of the male.’”’ The other three months 
are probably winter. No observations on the length of this 
cycle are given. 
Sobotta (95) is the only embryologist supposedly timing 
his stages from an oestrous period other than that following 
parturition, and he believed the duration of the cycle to be 
equal to the gestation period, i.e., 20+ days. Apparently all 
writers up until as late as 1916-17 have followed Sobotta in 
this particular. 
Danforth in unpublished observations in 1914-15 attempted 
to check the length of the cycle by tabulating the number of 
days between two successive litters and computing the greatest 
common divisor of these intervals. In his records of sixty-six 
animals, twenty-three were excluded because of the possible 
complication due to the mother lactating while pregnant. The 
greatest common divisor of the modes of his curve was found to 
be 4 to 6 days. This evidence is inconclusive, but it at least 
casts doubt on the existence of a uniform oestrous cycle of 20+ 
days. . 
H. P. Smith (17) attempted to approach the problem from a 
consideration of the ovarian cycle. This method permits of 
only one reliable observation, other than that of parturition, on 
one animal, namely, that made on histological examination of 
the ovaries after death. Conclusions must be drawn from a 
series of animals on each of which only one observation is made. 
The individual variation among even litter mates is so great as 
to make this method very inaccurate although a large number of 
animals be used. By this method Smith was led to conclude 
that the oestrous cycle is one of great variability, averaging 
seventeen and one-half days. He began the collection of his 
series at parturition, and his results are therefore really the 
recovery time of the ovaries—the time required for a resumption 
