DIOESTROUS CYCLE IN THE GUINEA-PIG 227 



troduced a small nasal speculum which facilitates a clear view 

 of the interior and a smear is made of any fluid that may be 

 present. 



A microscopic study of these vaginal fluids, to be described 

 in the following pages, has shown that the guinea-pig possesses 

 a perfectly regular and typical dioestrous cycle. And further, 

 the surprising fact that the composition of the fluids is exactly 

 comparable to the menstrual fluid taken from so high a mammal 

 as the monkey. Heape, ('99), states that the menstrual fluid of 

 the monkey contains a mucous secretion of the uterine glands, 

 blood corpuscles, particles of stroma and epithelium from the 

 uterus and the vagina and leucocytes. All of these elements are 

 present in the fluid from the vagina of the guinea-pig during 

 heat though the relative amounts differ from those in the 

 monkey and the fluid is rarely sufficiently abundant to be recog- 

 nized on the vulva. 



The great advantage of this simple method of examination 

 for the study of the oestrous cycle in these mammals which show 

 no external signs of heat is evident, and we trust that the method 

 may prove useful to those who find it necessary or desirable to 

 know accurately the sexual periods in animals used f o ' experi- 

 mental breeding. 



Having begun a study of the vaginal smears from guinea-pigs 

 we have been led to a more complete consideration of the uterine 

 changes which alter the composition of these smears, and finally 

 to an in^'estigation of the changes in the ovary and the process 

 of ovulation and corpus luteum formation which accompany the 

 activities on the part of the uterus. The present contribution 

 comprises the results of these investigations. 



2. CONSIDERATION OF THE LITERATURE ON OVULATION IN THE 



GUINEA-PIG 



It has been recognized for more than half a century that the 

 guinea-pig comes into heat very quickly after giving birth to a 

 litter of 3^oung. This period immediately following parturition 

 has been the starting point for the great majority of studies on 

 the sexual behavior of this animal and it has been demonstrated 



