244 CHARLES R. STOCKARD AND G. N. PAPANICOLAOU 



they are sexually active throughout the entire year with an 

 astonishingly regular return of their oestrous flow and breeding 

 reactions. 



A more careful consideration of the figures obtained during 

 the different months indicates, however, that there probably is a 

 small difference in the length of the sexual cycles during the warm 

 and the cold seasons. 



The curve shown in figure A indicates graphically this slight 

 fluctuation, operated animals are excluded. The lowest aver- 

 age 15.50 days, or the shortest oestrous cycles, was found in the 

 month of October, while the highest 16.14 days is shown during 

 Januarj^ The heavy line at 15.82 days indicates the mean 

 between these two extremes. It is probably not without sig- 

 nificance that the averages during the months December, Jan- 

 uary, February, March and April fall above the mean line, 

 while the averages during the months of May, June and October 

 are below the line. From the cases considered this indicates 

 that the length of the oestrous cycle is probably a little shorter 

 during the warm time of the year and a httle longer during the 

 cold weather. We must, however, admit that the number of 

 considered cases, as given in table 1, is actually small and these 

 slight seasonal variations may be more suggestive than demon- 

 strative in importance, yet there is certainly a striking consistency 

 in their arrangement. 



6. CYCLICAL CHANGES IN THE UTERUS AND VAGINA 



After having determined the regularity of the dioestrous cycle 

 in a number of virgin females, they were killed at different stages 

 of the oestrous period and their ovaries as well as pieces of the 

 uterus and vagina were carefully examined and then fixed and 

 preserved for microscopical study. The uterus and vagina 

 must be fixed in certain fluids to avoid shrinkage and a tearing 

 away of the epithelium from the wall. Bouin's fixing fluid has 

 proven most satisfactory for this purpose while the ovaries were 

 generally fixed with Zenker's fluid. 



During the dioestrum or resting period the uterus is lined by a 

 layer of cuboidal ciliated epithelium. Figure 10 shows a sec- 



