DIOESTROUS CYCLE IN THE GUINEA-PIG 233 



mals than in others. In some cases it can be hastened through certain 

 external factors, especially copulation, but in the large majority of 

 cases it occurs sooner or later even without a preceding copulation. 



He holds that eight days after ovulation large follicles are pres- 

 ent in the ovary but sometimes ovulation may not occur for 

 twenty or twenty-four days. 



The 'sexual peziod,' period between two ovulations, according 

 to Loeb lasts usually twenty to twenty-five days instead of being 

 about two weeks, the time necessary for mature folhcles to ap- 

 pear. This delay in ovulation in spite of the presence of mature 

 follicles within eleven to thirteen days, he beUeves is due to a 

 mechanism in the ovary which prolongs the cycle, the corpus 

 luteum begins this mechanism. The corpus luteum degenerates 

 after a period of growth lasting from seventeen to twenty days 

 and thus ovulation occurs about once in three weeks. We shall 

 show beyond by a demonstration of the oestrous cycles, that 

 Loeb's deductions drawn from studies of the histology of the 

 ovary ai*e incorrect and, therefore, cannot be employed for 

 determining the ovulation cycles in these animals. 



Loeb further finds that when the corpus luteum is cut out 

 immediately after an ovulation, the next ovulation occurs soon 

 after mature follicles are developed— about thirteen to fifteen 

 days. Under these conditions the normal sexual cycle is re- 

 established — but even here his periods are not exact being 

 somewhat shorter than are actually normal. 



The very varied time results obtained by Loeb may be given 

 as follows: First, no ovulation has been found under normal 

 conditions before the fifteenth day after the last copulation. 

 Second, in a group of thirty-eight guinea-pigs killed fourteen 

 days and eighteen hours and nineteen days and fifteen hours 

 after the last copulation — one had o\ailated about the sixteenth 

 day, another the eighteenth and another at the nineteenth day, 

 while the remaining thirty-five had not yet ovulated. Third, 

 in a lot of twenty-two guinea-pigs, twenty to twenty-six days 

 after the last copulation, one had supposedly ovulated at the 

 eighteenth day, four at the nineteenth day, one at the nine- 

 teenth to twentieth day, one at the twenty-third and one at the 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 2 



