HORMONES AND REPRODUCTION 71 



lasts for about 12 days, that of corpus luteum activity for, 

 about 12 more days, and that of tissue destruction in 

 the uterus (the menstrual period) for about 4 days. 

 However, man is peculiar in that the widest variation 

 exists in the time of ovulation within the cycle, although 

 in any one individual this time is perhaps fairly constant. 

 Thus the follicular and luteal phases may be longer or 

 shorter than 12 days. A case has even been described in 

 which ovulation took place regularly during the men- 

 strual period, ^s which was perhaps due to the failure of 

 the corpus luteum to secrete progesterone and so to 

 delay uterine breakdown. 



9. THE SEASONAL CYCLE 



A seasonal cycle of reproduction is found in most 

 vertebrates and in both sexes, and like the oestrous cycle 

 its internal control has been thought to be centred in the 

 anterior pituitary gland (but see p. 102). Marked changes 

 have been described in the histology of this gland at the 

 onset of the breeding season, one of the best surveys 

 being that of Moore, ^*^ who studied the ground squirrel. 

 During anoestrus the anterior lobe of the pituitary is 

 reduced in volume, its cells consist mainly of small 

 chromophobes, and no gonadotropic hormone can be 

 detected. With the approach of spring the anterior 

 pituitary increases in size, there is a marked production 

 of basophil cells, and the presence of gonadotropic 

 hormone can be demonstrated. At the close of the 

 breeding season the reverse changes occur. Similar 

 variations in the anterior pituitary have been described 

 in the case of the dog,^^^ the wood-chuck, ^^^ and the 

 Virginian oppossum.^^^ 



With the formation of the basophil cells and the con- 

 sequent rise in the output of gonadotropic hormone, the 

 testes and ovaries are stimulated to develop. In the 

 testes this is seen in an increase in the number of 

 glandular interstitial cells, which begin to secrete the 



