DIOESTROUS CYCLE IN THE GUINEA-PIG 259 



found in the ovaries examined, the sHght variations are all very 

 probably within the limits of error. We also believe that Loeb 

 has been misled by the application of similar methods in calcu- 

 lating the normal sexual periods in these animals. 



In order to test the influence of the removal of the corpora 

 lutea on the following ovulation time, one must first definitely 

 establish a normal o\iilation period. Since this was not done we 

 are forced to acknowledge that Loeb's experiments do not 

 demonstrate the importance of the corpus luteum in regulating 

 the ovulation process, though he must be credited for having 

 definitely attacked the problem experimentally. Some doubt 

 will also exist in the minds of those who have attempted the 

 operation as to whether all of the corpora lutea are often to be 

 removed from the ovary while it is in position in the abdomen. 



We are not at all opposed to admitting the probability that the 

 removal of the corpus luteum may shorten the usual sexual cycle. 

 In fact such a discovery would accord Avith our notions of the 

 function of the corpus luteum. We feel further that the present 

 study has established the existence of a definite normal oestrous 

 cycle and this knowledge makes the experimental analysis of the 

 influence of the corpus luteum much more readily approached. 



The knowledge of a typical and regular sexual cycle in the 

 guinea-pigs as here demonstrated, paves the way for a better 

 and more uniform understanding of the oestrous conditions pre- 

 vailing in the different classes of mammals. All cases that have 

 been studied with sufficient care give evidence at least of some 

 rhythmical activity. The absence of external signs of oestrus 

 in a great number of mammals, one of which was the guinea-pig, 

 is the most e\'ident cause of a lack of understanding of their 

 sexual periodicity. It is to be hoped that the application of the 

 simple method of examination of the vaginal fluid used in the 

 present study may enable workers to readily obtain a clearer 

 understanding of the sexual acti\'ities of other commonly used 

 laboratory animals as well as mammals in general, since such 

 information is of the greatest value in all exact experimental 

 breeding. 



