THE OVARY AS TIMEPIECE 



How menstruation is brought about by the ovarian hor- 

 mones, and what purpose it may serve, we shall discuss in 

 Chapter VI. 



The vaginal cycle. Obviously, the periodic ripening of the 

 ovarian follicles sets in action large forces that can alter the 

 state of other organs, change the reactions of the body, and 

 determine the behavior of the female animal. By means of the 

 ovarian hormones released at this time, there are ( as we shall 

 see) cyclic changes in the whole reproductive tract. Not only 

 is the lining of the uterus modified, but the Fallopian tube, 

 the vagina, and in some species even the external genital 

 organs also take part in these rhythmic alterations. This fact 

 led to the discovery, twenty-five years ago, of an extraor- 

 dinarily useful method of research, which greatly increased 

 our rate of progress in unraveling the problems in this field ; 

 and because it was made in the United States, helped put the 

 American investigators of this generation in the forefront. 



To grasp the importance of this discovery, we must remem- 

 ber in the first place how very helpful to science in general 

 are the rat, mouse, and guinea pig. These little rodents are 

 hardy, inexpensive, and easy to feed, house, and handle in the 

 laboratory. Their small size is also a great advantage when 

 experiments call for treatment with scarce or expensive hor- 

 mones and other chemical reagents. Unlike cats, they breed 

 freely in cages. Unlike dogs, they have rapid cycles, requiring 

 no long waits in the course of experimental study. Unfor- 

 tunately, among all the mammals they give the least conspicu- 

 ous signs of estrus. They show no genital swellings like the 

 bitch, no excitement like the sow. They normally mate only 

 at night, and to be certain they are in estrus, the investigator 

 had to put them with males and sit up all night watching them, 

 or else use roundabout methods of observation which were not 

 perfectly reliable. As an illustration of the difficulties, I recall 

 that before 1917 a distinguished biologist who was working 

 chiefly with guinea pigs supposed their cycle to be 21 days 



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