THE RHYTHM OF GONADAL FUNCTION 579 



offspring. In these latter animals if fertilization does not result at one of 

 these ovulatioii periods, a long period of inactivity or rest may result, the 

 period which Heape has termed the anoestrum ; on the other hand, it is open 

 to question whether in all cases of animals with shorter cyclic ovulation and 

 when fertilization is prevented, any true resting period, anoestrum or dioes- 

 trum, can be said to exist. In some cases of cyclic ovulation a resting period 

 certainly does not exist (e. g., the rat and man), for no sooner does the 

 functional life span of one corpus luteum draw to a close than a new cycle 

 of ovarian and other genital changes is begun. 7 



(Estrus the Precursor of Ovulation in the Mammalia. Certain bodily 

 changes associated with this cyclic ovarian activity are probably invariably 

 present, are often very pronounced and in some cases are preliminary or 

 anticipatory in time, so that the approaching ovulation may be predicted 

 with certainty. These anticipatory events constitute the phenomena, of 

 pro-oestrus and oestrus, knowledge of which we owe above all to the long 

 continued studies of Walter Heape of Cambridge and, more recently, to 

 those of F. H. A. Marshall. Thus animals possessing a cyclic ovulation 

 show a cyclic oestrus and the time of ovulation always occupies a definite 

 position in the so-called cestrous cycle. 



Succession of Events in the (Estrous Cycle. Since the phenomena, of 

 pro-oestrus and oestrus are somewhat different in different forms and are 

 completely understood in but few forms, it will be well to give here but a 

 single typical example of the succession of these events in an oestrous cycle. 

 Stockard and Papanicolaou have given us a most admirable and detailed 

 account of cyclic oestrus and ovulation in the guinea pig and Long and 

 Evans have conducted similar long-continued studies on the sequence of 

 events in the rat. To breeders of animals and veterinarians some of these 

 phenomena have long been familiar, for example, the twenty-one day cycle 

 in the cow. Corner has recently furnished details of the events in oestrus 

 in the sow. I shall describe the series of changes in the oestrous cycle 

 of the rat. 



Changes in the ovary and essentially the growth and transformation 

 of follicles provoke changes in the entire reproductive tube from the 

 fibrinated end of the oviduct to the external vaginal orifice. We are only 

 beginning to understand these changes in their entirety. Until very 

 recently they were thought to concern the uterus alone. This is probably 

 duo to the predominant role of uterine changes, causing menstruation, in 

 man and the primates. The tubal changes are still obscure, but it would 



T In other animals, for example the cat (Longley), ovulation is not spontaneous 

 but is induced by coition and if opportunity for the latter is not afforded, degeneration 

 of the ripe Graafian follicles occurs and corpora lutea are not formed. It is not yet 

 possible,, however, to greatly enlarge the list of mammals in this class as Ancel and 

 Bouin have done, for many of their examples have subsequently been proven to con- 

 stitute ideal examples of the spontaneously ovulating class, e.g., the guinea pig and rat. 



