DIOESTROUS CYCLE IN THE GUINEA-PIG 237 



first stage having a mucous secretion, a second stage the cheese- 

 hke secretion, a third stage with the fluid becoming serous and a 

 fourth stage, not always recognized, during which a bloody dis- 

 charge is present. The duration of these several stages is sub- 

 ject in the different animals to individual variations. The first 

 stage, however, is generally longest and lasts from six to twelve 

 hours or even more and during this time there is a gradually in- 

 creasing quantity of the mucous secretion which at its height is 

 very abundant and fills the entire lumen of the vagina. The 

 second stage is shorter, lasting from two to four hours, and 

 passes gradually over into the third stage which lasts from four 

 to six hours. The fourth stage is the shortest, only about one 

 to two hours long, and for this reason it is often missed in exam- 

 ining the animals during the periods. It is also possible, as 

 mentioned above, that the fourth stage may not typically exist 

 in all individuals and the quantity of blood present is very dif- 

 ferent in the different specimens. The succession in which these 

 stages follow one another is remarkably definite. We have 

 never observed any change in the typical sequence of the stages 

 and the time consumed by the entire process is generally as 

 stated about twenty-four hours. 



A macroscopical examination of the uterus and vagina during 

 this period of sexual activity shows the entire genital tract to be 

 congested. The vessels to the ovary, uterus and vagina are 

 large and conspicuous, the uterine horns and the vagina are 

 slightly swollen and inflamed. However, as soon as this short 

 period of activity is over, the congestion disappears and the 

 uterus and vagina take again their normal pale aspect. At the 

 same time the vaginal fluid diminishes and the vagina, especially 

 during the first week after this sexual acti\dt3^, is as clean as 

 possible sho^\dng none of the secretion. The external vaginal 

 orifice, which during the period of activity is more or less open 

 actually shomng in a few cases a little fluid or some blood, closes 

 and becomes less accessible after the period. 



During the second week follo^Aing oestrus a little mucous dis- 

 charge begins to appear in the vagina and increases progressively 

 indicating that the new period of activity is nearer and nearer 



