THE KHYTHM OF GONADAL FUNCTION 581 



period proper, which shows itself by the replacement in the vaginal smear 

 of the small, nucleated epithelial cells of the pro-oestrus by large, non- 

 nucleated, transparent, cornified cells. Animals invariably mate during 

 this period, the approximate length of which is twelve hours. During it 

 the uterine hyperemia and distention reach their greatest expression and 

 recede. Large Graafiaii follicles are by this time found in the ovary and 

 in some instances the first maturation spindle may be formed. 



The next, or postoestrous period (eighteen hours in length) is similarly 

 characterized by a continuation and accentuation of the vaginal smear 

 picture of the preceding stage, for the desquamated cornified cells now 

 appear in greater numbers. In the ovary the final ripening changes pre- 

 liminary to ovulation take place, the first maturation mitosis and in most 

 cases ovulation itself taking place before the end of the period. Im- 

 mediately consequent upon ovulation the ovarian portion of the oviduct 

 becomes distended with a clear fluid which had previously been secreted 

 into the periovarial space or bursa ovarica. In the ovary itself corpora 

 lutea are rapidly formed out of the follicles which have shed their eggs 

 and while for perhaps half a day there is some central follicular cavity 

 not yet closed up or occupied by the new lutein tissue, when one day has 

 transpired this is no longer true and the corpora lutea. are apparently 

 completely formed structures. 



In the postovulatory period it is the corpora lutea which are re- 

 sponsible for further marked changes which now regularly occur in the 

 genitalia of some other mammals. They produce distinct changes in the 

 rats' mammary gland. The mammary ducts, which show mild growth 

 changes during the ante-ovulatory periods of the oestrous cycle, take on a 

 special activity, branching rapidly, after the incidence of ovulation, so 

 that a considerable complexity of the ducts now results, a complexity 

 which is not resolved by degeneration until after the occurrence of the 

 next succeeding pro-oestrus, more than fifty-four hours afterwards. Thus, 

 cyclic growth and regression changes occur in the mammae of isolated 

 virginal animals, changes which owe their fullest expression to hormons 

 produced by corpora lutea. In animals in which ovulation is not spon- 

 taneous but is produced naturally only by the act of coition, these growth 

 changes in the mammary apparatus do not occur unless ovulation and the 

 consequent corpus formation have resulted. Ancel and Bouin have proven 

 this decisively in the rabbit, where ovulation was induced by permitting 

 copulation with vasectomized males or by puncture of ripe Graafiaii 

 vesicles. In this animal the formation and function of corpora lutea entail 

 specific pregravid changes in the uterus, changes in the direction of an 

 increase of both muscular and epithelial elements, the latter forming 

 glandular invaginations, changes which are at their height from the 

 seventh to the tenth day and which regress from the fourteenth to twenty- 

 fifth day, in nice correspondence with the degeneration of the corpus 



