THE HORMONES IN HUMAN REPRODUCTION 



males to fertilize them. Such a season of mating is well-nigh 

 universal in nature, though fortunately not all the denizens of 

 earth react as violently as the bison. The rhythm of sex is 

 manifested in infinite variety, assuming every aspect ; now to 

 our human eyes tensely dramatic, now gently romantic, now 

 bestial or merely matter-of-fact, sometimes even comical. 



In springtime two robins nest beneath my window, and 

 soon a group of eggs in the nest gives evidence that the ovaries 

 of the female bird have responded to the rhythm of the year. 

 Twice a year my neighbor's spaniel gives unmistakable signs 

 that she is ready to mate, and I know, being an anatomist, 

 that if we could inspect her ovaries we should see the Graafian 

 follicles enlarging, and the microscope would show us a crop 

 of ripening eggs. 



The ovary as a timepiece runs at curiously different rates 

 in different animals. Many creatures living wild, both plants 

 and animals, necessarily time their breeding with the seasons 

 of the year, because their offspring must begin life when con- 

 ditions of temperature, shelter, light and food are most favor- 

 able. Hence the vernal growth of plants and all the annual 

 breeding seasons of animals such as those of migratory birds 

 and of the fish, salmon and shad for example, that swarm into 

 the bays and rivers every spring, teeming with roe and milt 

 and seeking a sheltered place in which to spawn. Many marine 

 plants and animals have reproductive cycles controlled by the 

 tides and therefore breed at intervals of a month or multiples 

 of a month. There are seaweeds, for instance, that fruit only 

 on the highest tides, and worms that breed at particular 

 phases of the moon. The Japanese palolo, an annelid worm 

 living on the sea bottom, swarms to the surface to breed on 

 four nights of every year, namely on the new moon and the 

 full moon of October and November (Appendix II, note 2). 



In the course of evolution, however, many animals have 

 adopted cycles not directly related to the yearly seasons or 

 the tides. Some of these are domesticated species which man 



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