m. 645] FECU. 



349 



the guinea pig by making microscopic examination of the 

 material found in the vagina. They found that " Guinea 

 pigs kept in a state of domestication and under steady 

 environmental conditions possess a regular dioestrous cy- 

 cle, repeating itself in non-pregnant females about every 

 sixteen days throughout the entire year, with probably 

 small and insignificant variations during the different 

 seasons. Each period of sexual activity lasts about 24 

 hours and is characterized by the presence of a definite 

 vaginal fluid which is not sufficiently abundant to be read- 

 ily detected on the vulva, but is easily observed by an 

 examination of the interior of the vagina." They added 

 that macroscopic signs of heat are unreliable. 



In my work it was found impractical to determine the 

 existence of heat microscopically and the knowledge that 

 heat should recur about every fifteen days furnished a 

 starting point. Each female was given a number and en- 

 tered on an individual record sheet giving the following 

 data: 



Date and hour of attempted mating. 

 Eesult of attempted mating. 



Each time the animal came into heat the record showed : 

 Whether heat was recognizable by macroscopic examina- 

 tion. 



Number of days since last heat. 

 Number of hours since the first successful coitus. 

 Number of hours that external signs of heat could be ob- 

 served by examination. 

 Matings were attempted daily, whether the animal was 

 supposed to be pregnant or not. The males were intro- 

 duced into the pens with the females regardless of whether 

 or not the latter were thought to be in heat, and they were 

 allowed to remain with the females from five to fifteen 

 minutes. It was easy to follow the dioestrous cycle of any 

 individual animal. A glance at the guinea pig's record 

 each day showed the number of days since the last heat, 

 and, knowing that heat should return about the fifteenth 

 day, it was practically impossible for it to come and go 

 unnoticed unless it recurred altogether irregularly. We 



