260 CHARLES R. STOCKARD AND G. N. PAPANICOLAOU 



The typical oestrous cycles are probably more regularly ex- 

 pressed among mammals living in a state of domestication, and 

 consequently under steady environmental conditions, than 

 among their relatives living in the wild, where the existence of 

 great disturbing factors, especially variations in food and tem- 

 perature conditions, may tend to modify their behavior. The 

 evidence of such modification by these disturbing factors is the 

 existence in most mammals of differences in their sexual be- 

 havior during the different seasons of the year. Such seasonal 

 variations are frequently lost under uniform conditions of tem- 

 perature and feeding as is the case with rabbits, and also with 

 guinea-pigs if these show seasonal changes in their native wild. 



It has been reported by some investigators, Rubaschkin and 

 others, that guinea-pigs in captivity breed less frequently in 

 winter than during the warmer months, though they may be- 

 come pregnant at any season. Such results are probably due 

 to a failure to keep the animals properly warm during winter. 



Guinea-pigs under the uniform conditions of our experiments 

 do not show any apparent changes in their sexual rhythm with 

 the seasons, but as indicated on previous pages, it is probable 

 that their sexual cycle is a little shorter during the summer than 

 in winter, yet even this difference does nOt seem to be very 

 definitely expressed. 



9. SUMMARY 



The above description of the details of the oestrous cycle in 

 the guinea-pig may be briefly summarized as follows: 



1. Guinea-pigs kept in a state of domestication and under 

 steady environmental conditions possess a regular dioestrous 

 cycle repeating itself in non-pregnant females about every six- 

 teen days throughout the entire year with probably small and 

 insignificant variations during the different seasons. 



2. During each cycle typically corresponding changes are 

 occurring in the vagina, the uterus, and the ovary; a given stage 

 in one of these organs closely accompanying parallel stages in 

 the other two. 



3. Each period of sexual activity lasts about twenty-four 

 hours and is characterized by the presence of a definite vaginal 



