36 



INTERNAL SECRETIONS OF THE OVARY 



The vaginal cycle of the guinea-pig is remarkable for the fact 

 that during the dioestrous interval the vaginal orifice is normally 

 closed by an epithelial membrane. This is ruptured at the 

 approach of procestrus by the turgidity of the vulva. Stockard 

 and Papanicolaou have divided procestrus and oestrus (which 

 together last about twenty-four hours) into four stages according 

 to the nature of the vaginal contents (Table 2). 



Table 2. — Vaginal changes during (Estrus 



IN THE GuINEA-PiG 



Copulation, accompanied by the formation of the vaginal plug, 

 takes place during the first stage, and the corpora lutea are fully 

 formed four to five days later. Stockard and Papanicolaou con- 

 sider the breakdown of the vaginal epithelium to be due to the 

 removal of the protective influence of the corpora lutea of the 

 previous ovulation, and to be homologous with human mens- 

 truation. The time relation of ovulation is, however, radically 

 different from that found in the human. 



Loeb and Hesselberg (402) have described a growth cycle in 

 the mammary glands of the non-pregnant guinea-pig corres- 

 ponding with the dioestrous cycle, a burst of growth accom- 

 panying each oestrus. Following oestrus, the glands regress and 

 there is no reactivation during the luteal phase of the cycle. 



