OESTROUS CYCLE 19 



fed and tended group, Greenman and Duhring ('23) have had 

 exceptional females cast their litters at 55 and 57 days of age. 

 These rats therefore became pregnant at 33 and 35 days 

 respectively. 



In the breeding Albino it is found that impregnation most 

 readily follows 24 hours after a litter has been cast. This ac- 

 cords with the time of ovulation (Kirkham, '10; Sobotta and 

 Burckhard, '10; Kirkham and Burr, '13). During the breeding 

 period the female ovulates at intervals of 5 days or slightly 

 less (Long and Evans, '22), but only from April to October do 

 the females regularly ovulate 15-24 hours after parturition. 



The menopause commonly appears at the age of 15-18 months 

 (equivalent to 38-45 years in man). Particularly good condi- 

 tions of food and care tend to delay the appearance of the 

 menopause (Greenman and Duhring '23). 



Oestrous cycle. 8 Long and Evans ('22) observed the oestrous 

 cycle to extend from three to thirteen days or more, with a 

 well marked tendency to be about five days (4.8) table 2. 



In a later study Evans ('24 MS.) determined the proportion 

 of the short or normal cycles in two colonies of Albinos as com- 

 pared with the gray and white cross of Long and Evans, with 

 which the original studies were made. The results are given in 

 table 2a and show a very fair agreement among the three series. 



By using the method of vaginal smears, loc. cit. p. 41, five 

 stages in the oestrous cycle have been determined as shown in 

 table 3. 



In table 4 is given the number of hours for these several 

 stages in a cycle of something over four days. 



The fifth stage or dioestrous interval in these instances ex- 

 tends for 48 hours or more. 



The length of the first post partum oestrous cycle when suck- 

 ling is prevented, was found to be from 5-7 days, and when 

 the young were nursed less than 12 hours, somewhat less than 

 seven days. 



8 Under the title of ''The oestrous cycle in the rat and its associated phenom- 

 ena," Long and Evans ('22) have recorded a very complete series of observations. 

 For this study pied rats (black and white) were used, but the results are applicable 

 to the Albino and several of the tables and conclusions are here cited. 



