36 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



cycle typically corresponding changes are occurring in the vagina, the 

 uterus, and the ovary. Each period of sexual activity lasts about twenty- 

 four hours, and is characterized by a vaginal fluid changing in com- 

 position with the several stages. These changes are described in detail. 

 Ovulation seems to occur spontaneously in every heat. During the 

 dioestrum there is very little fluid in the vagina. A marked correlation 

 exists between the cestrous changes in the uterus and the developmental' 

 cycle of the corpora lutea. Tt looks as if the secretion from the corpora 

 lutea exerted a protective influence over the uterus and vagina, while its 

 absence permitted the breaking down and degeneration of the uterine 

 epithelium typical of the heat period, 



Post-oestrous Changes in Dog.* ^ F. H. A. Marshall and E. T. 

 Hainan have studied the post-oestrous changes occurring in the genera- 

 tive organs and mammary glands of the non-pregnant dog. There is a 

 pronounced post-oestrous development under the influence of the corpora 

 lutea, there being a definite pseudo-pregnant period. Retrogressive 

 changes do not set in with any of these organs until about thirty days 

 after ovulation, and in the case of the mammary glands a somewhat later 

 period. The developmental changes are of a similar kind to those 

 taking place during pregnancy, but do not reach the same degree of 

 development. The entire series of changes is physiologically homo- 

 logous with the series shown by the uterus and mammary glands of 

 the pseudo-pregnant rabbit and marsupial cat. 



The relatively long persistence of the corpora lutea in the bitcn i& 

 probably correlated with the monoestrous habit. This persistence, which 

 is possibly greater in some individuals than in others, elircidates the not 

 uncommon phenomenon of bitches which have not been impregnated 

 secreting milk at or near the end of the pseudo-pregnant period. 



The changes which occur in the generative organs and mammary 



glands after oestrus are now brought into relation with the rest of the 



oestrous cycle. The complete cycle in the bitch may be summarized in 



the following scheme : — 



Pro-cestrum 



>^ 

 CEstrus 



Pregnancy ^ \ Pseudo-pregnancy 

 Ancestrum 



The terms pro-oestrum, oestrus, and ancestrum are used as defined by 

 Heape. The first part of the ancestrum is generally occupied by the 

 nursing or lactation period, but in the case of animals which have 

 experienced pseudo-pregnant conditions the lactation period is usually 

 only very imperfectly represented. The metoestrous period must now be 

 regarded as unrepresented in the bitch. It exists in those animals which 

 do not experience pseudo-pregnancy (e.g. in those rabbits in whick 

 corpora lutea are not formed after oestrus). 



* Proc. Roy. Soc, Series B, Ixxxix. (1917) pp. 546-59 (3 pis.). 



