OBSERVATIONS ON THE PERIOD OF GESTATION IN 
WHITE MICE 
J. FRANK DANIEL 
Instructor in Comparative Anatomy, University of California 
From the Zoological Laboratory, University of Michigan 
Preliminary to a study which I am making on white mice, I have 
found it necessary to reexamine in detail their period of gestation. 
In this examination I have ascertained facts which, so far as I 
am aware, have not been previously described. 
The general statement that the period of gestation in white 
mice is 21 days,^ needs considerable modification. Such may be 
the case, but the likelihood is against it. In fact it may be said 
that no unqualified statement can be made as to a definite period 
of gestation for all mice — since this period depends upon the 
state of the individual mouse. 
My plan of study, which has consisted of isolating the female 
immediately upon an observed copulation, and of keeping her 
apart from the male until the birth of her young — has been 
greatly facilitated by the use of observation cases of glass, in 
which the mice were kept under almost constant attention during 
the entire period of gestation.- Under this condition, by observ- 
ing the moment of copulation and the time of birth, I have been 
able to determine in many cases the period correct to the hour. 
1 Allen, Glover M., 1904. The Heredity of Coat Color in Mice. Proc. Am. Acad, 
of Arts and Sciences, 40, no. 2 (p. 70). 
2 Strictly speaking, the period of gestation is that interval of time elapsing be- 
tween the act of fertilization and birth. In this paper it will be noted however, 
that I have used the term rather broadly to cover the period between the act of 
copulation and birth. 
