THE OVARY AS TIMEPIECE 



the cycle are due to an interplay or see-saw action between the 

 hormones of the ovary and of the hypophysis or pituitary 

 gland. To make this clear let us continue to anticipate the 

 next chapter by postulating that the ovary produces an 

 estrogenic hormone that acts upon the uterus and the rest 

 of the reproductive tract, producing among other effects the 

 estrous changes of the vagina referred to above. Another 

 fundamental fact is that the whole activity of the ovary, 

 including the production of the estrogenic hormone, is under 

 the control of the pituitary gland (see Plate XIX and Fig. 

 20). If this remarkable bit of glandular tissue is removed, the 

 ovary quits functioning altogether. By chemical extraction 

 the pituitary yields hormonal substances which can restore 

 ovarian function in the absence of the pituitary gland, or 

 cause the ovary of the immature animal to grow and begin 

 functioning. It has been shown, however, that if we inject 

 the ovarian estrogenic hormone into a female animal, the 

 pituitary hormones that stimulate the ovary are reduced in 

 amount. Here we have, in all probability, the fundamental 

 mechanism of the ovarian timepiece. The pituitary stimulates 

 the ovary, but the latter then sends out its estrogenic hor- 

 mone and this depresses the pituitary. Then the ovary loses 

 pituitary support. Up goes the pituitary again like the other 

 end of a see-saw. There must of course be other factors in the 

 situation, for such a mechanism, like the see-saw, will come 

 to balance unless it gets a push from time to time. In Chapter 

 VI we shall return to this subject after a more detailed ac- 

 count of the ovarian hormones. 



i 76 } 



