84 INFLUENCE OF HORMONES 



of the nuptial plumage, but copulation does not occur 

 till spring is advanced. The investigation here 

 considered was made upon specimens of semi- 

 domesticated Anas boscas, such as are kept in London 

 parks and supplied from game farms. The testes 

 attain their maximum size during the breeding 

 season end of March or beginning of April. At 

 this time each organ is almost as large as a pigeon's 

 egg, is very soft, and the liquid exuding from it when 

 cut is swarming with spermatozoa. The bird is of 

 course in full nuptial plumage. By the end of May, 

 although the plumage is unchanged, the testes have 

 diminished to the size of a haricot bean, and sper- 

 matogenesis has ceased. They diminish still further 

 during June, July, and August, and acquire a yellow 

 or brownish colour, while microscopically there is no 

 sign of activity in the spermatic cells. The change 

 from nuptial plumage to eclipse takes place between 

 the beginning of June and the middle of July. The 

 reappearance of the nuptial plumage takes place in 

 the month of September, and while this process 

 takes place there is no sign of change or renewed 

 activity in the testes. During October and November, 

 when the brilliant plumage is fully developed, the 

 testes increase slowly in size but remain yellow and 

 firm and exude no liquid on incision. Spermato- 

 genesis does not commence until the end of November 

 or beginning of December. The testes increase 

 greatly in size in January and February, and again 

 reach their maximum size by the end of March. It 

 is shown, therefore, that the loss of the nuptiaJ 

 plumage takes place in June when spermatogenesis 

 has ceased and the testes are diminishing in size, 

 but the redevelopment of this plumage takes place 



