DIOESTKOUS CYCLE IN THE GUINEA-PIG 241 



atypical cells there may exist isolated cells of the first or of the 

 second stage type. 



At the fourteenth and fifteenth day the number of first stage 

 cells already described begins to increase gradually and the 

 groA\ing proportion of these cells indicates the approaching new- 

 period of heat. 



5. THE OESTROUS RHYTHM 



The periodical return of a typical flow showing the above de- 

 scribed macroscopical and microscopical details, was found to be 

 very regular in twenty-six virgin females examined during dif- 

 ferent seasons of the year. Table I shows the results of this ex- 

 amination. As this table indicates, all the fema^les examined 

 were virgin thus eliminating any chance of modification which- 

 might be due to the act of copulation. Their ages ranged be- 

 tW'een three and a half and fifteen and a half months during the 

 time of examination. The female guinea-pig is sexually mature 

 at about three months old. Almost every animal, as the table 

 shows, was examined for a length of time covering several oestrus 

 periods. In the sixty-seven periods examined altogether the 

 vaginal flow returned regularly every fifteen to seventeen days 

 with an average of 15.73 days interval between the beginning of 

 periods. 



This table contains nine oestrus periods for operated animals 

 from which one ovary was removed. The operation w^as done 

 to determine w^hether any decided alteration in the oestrus would 

 result after the loss of one ovary. The animals 1080 9 and 

 1102 9 were semi-spayed during the time of examination given 

 in the table. In the animal 1080 ? the first heat period follow- 

 ing operation came at the sixteenth day after the la^st period but 

 in the animal 1102 9 the first heat period following operation 

 came on the fourteenth day after the last heat, a little earlier 

 than it should come inder normal conditions. The three heat 

 periods following this came, however, very regularly every 

 sixteenth day. 



The animals 867 9 , 923 9 , and 1069 9 were semi-spayed a 

 considerable time before the beginning of the examination and 



