DIOESTROUS CYCLE IN THE GUINEA-PIG 243 



all of these showed an interval of seventeen days between the 

 beginnings of the dioestrous cycles. The average of seven periods 

 in animals with only one ovary is 16.57, this being much higher 

 than the average of all the cases, which is 15.73 days. The aver- 

 age of the only two cases of first heat-periods after operation, on 

 the other hand, is lower than the general average 15.0. The num- 

 ber of cases is, however, entirely insufficient to warrant a con- 

 clusion, though suggestive for further investigation. It is prob- 

 able that when only one ovary exists, the period between ovula- 

 tions is a little longer than under normal conditions. The two 

 ovaries may alternate to a certain degree in their function or they 

 may share the entire task in a less exhaustive way than one 

 o\'ary is capable of doing. Semi-spayed females often have 

 large litters which might indicate that the single ovary matured 

 more foUicles than would have been its share should the other 

 ovary have been present. 



Eliminating from the general table the results obtained by the 

 examination of the semi-spayed animals, one finds an average of 

 15.65 days for the length of time from the beginning of one heat 

 period to the beginning of the next in all normal cases. This 

 we befieve to be the length of the oestrous cycle of the guinea-pig 

 under uniform conditions. 



Table 1 further shows the months during which these observa- 

 tions were made. The animals were examined during early 

 summer, fall, winter and spring and have shown at all seasons a 

 perfect regularity in the return of the heat periods. Their 

 oestrous cycle is certainly typically regular. The only months 

 during which the animals were not examined are July, August 

 and September. During the winter the guinea-pigs are kept 

 in a fairly well regulated warm temperature running about 70° 

 Falirenheit on an average. It may be possible that in the 

 wild state under natural conditions when the weather is cold 

 and food somewhat scarce, the heat periods may cease for a 

 season or become less frequent. Rubaschkin claimed that heat 

 ceased to recur after October when guinea-pigs were kept in a 

 cold place. But under the steadily favorable conditions in 

 which the guinea-pigs here considered are kept, it is certain that 



