354 THE AMERICAN NATURALIST [Vol.LVI 



As noticeable in the above chart, the lengthened dices- 

 trous periods are nearly exact multiples of 15, the nor- 

 mal period, thus showing that the cycle is definitely pe- 

 riodic as reported by Stockard and Papanicolaou, '176. 

 Long, '15, found that the oestrous cycle was prolonged by 

 inserting a glass rod in the vagina of the rat. He held 

 this prolongation to be diie to a stimulation of the cervix 

 of the uterus. Although I stimulated the uterus of guinea 

 pigs by means of a warm glass rod in three cases only, 

 heat returned in 15, 15 and 16 days, and I regret that I was 

 not able to extend this series of experiments in order to 

 obtain more data on this interesting phenomenon revealed 

 by Long in the rat. However, from the above table, it 

 is clear that copulation definitely prolongs the next oes- 

 trous cycle in the majority of cases. This may be due 

 to direct stimulation of the cervix of the uterus, as ex- 

 plained by Long, or implantation may have occurred, fol- 

 lowed by abortion or by absorption of the young concep- 

 tuses, in cases in which the period was greatly prolonged. 



Guinea pig No. 39 (see Table II) was mated two hours 

 after parturition, but no pregnancy followed. This ani- 

 mal was remated 12 days later, with resulting pregnancy. 

 This confirms a case reported by Eubasckhin, '05, in which 

 heat returned 10 days after parturition. Stockard and 

 Papanicolaou, in considering Eubasckhin 's report, re- 

 garded 10 or 12 days as too short a period to indicate the 

 return of heat. Nevertheless, in the case reported here 

 heat was unmistakable, and this animal which was mated 

 12 days after parturition became pregnant. I observed 

 heat to return in 12 days also in two other pigs. 



Young animals constantly in association with males 

 became pregnant at an earlier age than females isolated 

 from males. Of a litter containing 3 females and 1 

 male, two females were placed in separate cages a few 

 days after birth and the remaining male and female 

 were allowed to run together. At the age of 5 months, 

 the latter produced a litter. This indicates that the 

 mating of this pair occurred before the animals w^ere 

 three months old. Yet no ill effects of this early mating 

 or of the inbreeding could be detected in the offspring. 



