THE LIVING BIRD 41 



nective tissue, but only at certain times of the year. 

 These are termed interstitial cells. They are known 

 certainly to be the source of the internal secretions 

 of the mammalian testis and there is little doubt 

 that this is their function in birds. 



The ovary presents a somewhat different picture 

 (Fig. 7B). It lacks the tubular structure, the sex- 

 cell producing walls of the testicular tubules being 

 represented by the germinal epithelium, which, in 

 place of sperms, produces the eggs or ova. Whereas 

 millions of sperms may be demanded in a season, 

 the number of ova required is very small and many 

 start development only to be later resorbed. Dur- 

 ing the breeding season, such eggs as are destined 

 to mature, elaborate yolk material and become rela- 

 tively enormous. Each ovum is enclosed in a sac 

 of cells constituting the follicle, the various follicles 

 being bound together by connective tissue which, 

 with its blood vessels and lymphatics, forms the 

 ovarian stroma. Here develope glandular inter- 

 stitial cells at certain periods. As in the testis they 

 produce internal secretions. 



At the age of puberty, when the internal sex 

 organs are rapidly maturing, the human boy and 

 girl undergo a change. They grow, almost over- 

 night, into man and woman. But physical changes 

 go hand in hand with change of temperament, 



