72 VERTEBRATE REPRODUCTIVE CYCLES 



male hormone testosterone. In the first place- this 

 hormone acts on the testis itself to cause, or assist, the 

 process of spermatogenesis. In the second it circulates in 

 the blood stream to stimulate the growth of accessory 

 sexual organs, which include such structures as the 

 vesicle gland (the so-called seminal vesicle), and of the 

 secondary sexual characters, which are usually seen as 

 adornments of various kinds. 



In a male the secretion of testosterone is not cyclic, 

 but a balance is struck between it and the rate of secretion 

 of the pituitary gonadotropin. This balance remains 

 relatively steady until after all the spermatozoa have 

 been shed. Then the activity of the anterior pituitary 

 quickly ceases, and that of the testis perforce follows suit 

 as the glandular interstitial cells are reduced in numbers. 



The changes occurring at the onset of the breeding 

 season of a female mammal with only one oestrous cycle 

 in a year are those already described on p. 67. The only 

 difference is that no further cycle begins until after many 

 months of anoestrus. 



In the egg-laying vertebrates the effect of the seasonal 

 pituitary stimulus is two-fold. In the first place it causes 

 the growth of the primary oocytes by the deposition 

 within them of large quantities of yolk, and in the second 

 place it induces the secretion by the ovary of the oestro- 

 genic hormone. This hormone, by stimulating cell 

 division, causes the enlargement of the oviducts and of 

 any secondary sexual character which may be typical 

 of the species. 



10. THE PROBLEM OF INTERNAL 

 REPRODUCTIVE RHYTHMS 



These accounts of the reproductive cycles and of 

 their hormonal control have so far included no considera- 

 tion of the mechanisms which determine their timing. 

 This is actually a double problem since in the first place 

 there is the question of the method of timing of the 



