THE OESTROUS CYCLE IN THE MOUSE 307 
The criteria used in placing the animals are the following: 
1. The cell contents of the vaginal smear at death. 
2. The number of layers of vaginal epithelium unaffected by 
cornification or leucocytic infiltration. 
3. The position, with regard to the free surface of the epithe- 
lium, of the granular and horny layers when present. 
4, If ova are present in the uterine tubes, the segment in which 
they lie indicates the time of ovulation with regard to oestrus. 
5. The stage of development of the corpora lutea when present 
aids in a correct arrangement of the series. Their age is estimated 
by the position of the ova in the tubes, and compared with cor- 
pora lutea of pregnancy (recorded in column 7, table 1). 
The following table of animals killed for histological exami- 
nation represents a complete, closely spaced series. It seems 
convenient to begin at the middle of the dioestrous interval and 
return to the same point in the next cycle. 
The results of the histological examination of this series 
of organs can be most satisfactorily presented by means of 
a detailed description of typical animals (marked x in table 1) 
representing the midpoints of the five stages of the cycle. 
The dioestrous interval 
Stage D is best represented in this series by animal *2, a 
virgin mouse seven months old. She was killed after 53 days, 
observation, during which time nine complete cycles were re- 
corded, making the average duration 5.9 days. In six of these 
nine cycles pronounced external signs were apparent. The last 
four oestrous periods occurred 3 to 5, 11, 14 to 15, and 19 to 20 
days before her death. The record shows her to have been in 
an early day of the dioestrous interval after a cycle of from 7 to 
8 days’ duration marked by external signs and containing a three- 
day oestrous period. 
A gross examination of the internal genital organs was made 
immediately after opening the body cavity before there was any 
1 These underlying layers are very difficult to count in some of the phases, 
so that two numbers are used to express a limit of error. 
