CESTROUS CYCLE IN THE GUINEA-PIG. 227 



the corresponding vaginal or uterine stage. The description by 

 Loeb ('14), however, of the great leucocyte migration in the 

 wall of the uterus twelve to twenty-four hours after copulation 

 would indicate that the true "heat" or copulation occurs before 

 the beginning of the destructive changes in the uterus, and this 

 we now find to be true. Here and in the following section we 

 wish to point out just how the uterine changes seem to be 

 associated with the act of copulation, the retention of the sperm 

 in order to insure the fertilization of the eggs, and after this 

 the means of ridding the vagina of the excessive seminal accumu- 

 lation. 



A number of females have been placed with males while in 

 one or another of the above mentioned four stages, as well as 

 during different times of the dioestrum, or interval of sexual rest. 

 The results show that a copulation is never accomplished except 

 during the first stage of oestrus about twelve hours before the 

 second stage begins. Long ('19)- also finds copulation to take 

 place in the rat during the first stage. At this stage in the 

 guinea-pig the vagina contains a clear, foamy, saliva-like fluid in 

 which desquamated epithelial cells of the first type are present. 

 Differing from all other stages and times there are now no leuco- 

 cytes to be found in the vaginal fluid, compare our former Figs. 

 I and 2 with Figs. 5 and 6. Even during the resting period the 

 vagina contains some mucus but this is very scant and filled 

 with many leucocytes, being pus-like in appearance and con- 

 sistency. 



To locate even more accurately the normal time of copulation 

 the first stage may be subdivided into two shorter periods: a 

 preparatory interval, the early beginning of the first stage, when 

 the vagina is almost dry and contains only a scant amount of 

 loose cells of the first type; and, second, what may be designated 

 the true first stage, a more advanced period when the frothy 

 mucous secretion has already begun to accumulate. Copulation 

 takes place during this second phase of stage one and never during 

 the first. 



During stage one the vagina is characterized by two important 

 conditions, both of which contribute to the success of copulation. 

 In the first place the existence of a mucous secretion evidently 



