580 HERBERT M. EVANS 



appear certain at least that a secretory phase characterizes the epithelial 

 elements of the tube especially in its peripheral or ovarian portion at 

 about the time of and immediately succeeding ovulation though doubtful 

 whether the cells lose their cilia and later acquire the same (Moreau). 8 

 More remarkable are the changes which occur in the stratified squamous 

 epithelium of the vagina, for here in many animals a great increase in the 

 thickness and differentiation of the cell layers takes place as well as a mas- 

 sive desquamation of the surface sheets of cells. It is by reason of the lat- 

 ter fact and by recognition of the different types of desquamated cells 

 found in the vaginal lumen that in the case of some animals one can accu- 

 rately subdivide the stages of pro-oestrus and oestrus, as Stockard and 

 Papanicolaou have shown for the guinea pig and Long and Evans for 

 the rat. 



The entire o?strous cycle in the rat occupies ninety-six hours, i. e., this 

 is the time interval from one initiation of cestrous changes (i. e., from one 

 pro-oestrus) to the next recurrence of the same phenomena. In the case 

 of this animal it is possible by means of changes in the microscopic makeup 

 of smears from the fluid and cellular content of the vagina to distinguish 

 five different cell pictures. Three of these occur before the time of ovula- 

 tion and are related to certain peculiarities in the behavior of the animal, 

 permitting us to subdivide the total ante-ovulatory part of the cycle into 

 three periods, the pre-oestrous, oestrous, and postoestrous periods, as 

 shown in Fig. I. 9 The remaining two of the five components of the 

 cycle, as determined by the study of the smear, occur after ovulation, and 

 these have been combined in the above diagram into one period the post- 

 ovulatory period. 



In pro-oestrus the external genitals begin to show an evident swelling 

 and the cells in the vaginal smear no longer consist of leukocytes with 

 epithelial elements but consist of the latter alone, either singly or in con- 

 siderable sheets. Histological studies of the vaginal wall disclose the 

 origin of these cells from the surface of a suddenly thickening and differ- 

 entiating epithelium, which was previously but four cell layers in height 

 but is now double this dimension. Toward the end of this stage the 

 hyperemia which has begun to show in the muco'sal folds about the vaginal 

 orifice now involves the uterus, which in addition to its heightened vascu- 

 larity becomes distended with fluid. Rats will not mate during the pro- 

 oestrus. 



The next stage in the oestrous cycle of these small rodents is the oestrous 



8 But see Voinat, Schaffer, Hoehne and Troscher, as well as the older reports of 

 tubal menstruation recounted by Schaeffer. 



9 It will be noted that I do not follow Hill and O'Donoghue in denying the oc- 

 currence of any "postoestrous" and ante-ovulatory period in the eutherian mammals, 

 for ovulation is not immediately consequent upon oestrus in the rat or indeed in 

 several other eutheria known to me. 



