THE HORMONES IN HUMAN REPRODUCTION 



tite, and heightened sexual interest. If there is no boar present 

 she sniffs and nuzzles at the genitals of other sows, but if 

 there is a boar she promptly accepts mating and in the normal 

 course of events becomes pregnant. The cycles then cease until 

 the young are born. If she does not mate, or if the mating is 

 not fertile, cycles continue as usual every 3 weeks throughout 

 the year. 



These events in the life of the sow have been known to every 

 swineherd for ten thousand years, but nobody knew until the 

 present century just what is going on inside the animal every 

 21 days to stir her two hundred pounds of meat, bones, and 

 fat into three days of such specialized conduct. It turns out, 

 when we investigate the matter, that during the diestrous 

 phase of the cycle (the 21/2 weeks of no sexual activity) the 

 Graafian follicles in the ovaries are all small, with unripe eggs 

 in them. About two days before the onset of estrus, a crop of 

 follicles begins to grow. The ovaries of sows killed on the first 

 day of estrus contain large follicles with mature eggs. Late on 

 the second day we find that the follicles have ruptured and the 

 eggs are on their way down the oviducts. The follicles are 

 being converted into corpora lutea (see diagram, Fig. 15). 

 By the sixth or seventh day after ovulation the corpora lutea 

 are fully developed and (as we shall see in Chapter V)' are 

 at work producing their hormone, progesterone. This hor- 

 mone acts upon the uterus, altering its lining to make it ready 

 to receive the eggs, as a plowman tills the fields to receive the 

 seed. If the sow has mated while she was in estrus, the eggs 

 will be fertilized and they will settle down in the uterus and 

 develop there. Once the pregnancy is well established, the 

 cycles cease, possibly because a hormone produced by the 

 placenta (or rather the outer part of the embryonic tissues, 

 destined to form the placenta) signals the ovary, via the 

 pituitary gland, to stop development of follicles for the time 

 being. If not fertilized, the eggs will degenerate and go to 

 pieces; then the corpora lutea, no longer useful, begin to 



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