THE HORMONES IN HUMAN REPRODUCTION 



ance of the two hormones. Under the push-pull hypothesis it 

 is in fact easier to understand the long diestrous phase of 

 cycles like those of animals that have an annual cycle, when 

 (as we may suppose) the estrogenic and gonadotropic hor- 

 mones are balancing each other, than to explain what happens 

 to set the see-saw swinging again, or to bring on cycles, in 

 some animals, every few days or weeks. At present we can 

 only make vague conjectures about the possible role of other 

 hormones, e.g., progesterone or another pituitary hormone. 



Events of the cycle. To resume the main theme of our dis- 

 course, during all these changes in the ovary the uterus is, 

 of course, constantly under the influence of the ovarian hor- 

 mones. Even when there is a long anestrous interval between 

 one ovulation and the next, the ovary produces enough estro- 

 gen to protect the uterus from atrophy. As the follicles en- 

 large and ripen, there is a period of growth and development 

 of the lining of the uterus (endometrium). When the corpus 

 luteum is formed and begins to produce progesterone, the 

 uterine lining is rapidly brought into the progestational con- 

 dition, as described in Chapter V and illustrated in Plate 

 XVII. About one week is required to complete these changes. 

 The favorable environment thus prepared for the embryos 

 (pages 107-111) is maintained about one week longer, making 

 two weeks in all between the beginning and the end of the 

 active life of the corpus luteum. This is the progestational 

 phase of the cycle. If the animal mated while in estrus, the 

 embryos arrive in the uterus about the 4th day (later in 

 some species) and begin to attach themselves sometime be- 

 tween the 7th and the 13th day, according to the species. It 

 will be seen that the corpus luteum functions long enough to 

 give time for implantation of the embryos. If this occurs, 

 some sort of signal, probably via the pituitary gland, causes 

 the corpus luteum to survive and maintain the uterus in a 

 state favorable to early pregnancy. 



Retrogression of the uterine changes. We are considering 



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