476 



THE BIRDS 



xvi. 26- 



right is able to differentiate, but then forms not an ovary but a testis. 

 Complete sex reversal can thus occur, at least in some races of 

 domestic fowl, and the transformed bird may acquire cock plumage 

 and tread and fertilize hens (Fig. 287). Sex reversal rarely, if ever, 

 takes place in the opposite direction. We must suppose that there is 

 some switch over in the balance of male and female determining 



Fig. 287. Secondary sexual characters of the fowl (Gallus). 



Left cocks, right hens. 



A, normal; B, castrated; C, cock with implanted ovary and hen 



with implanted testis. (After Zawadowsky.) 



processes, taking place relatively early in the case of normal definitive 

 males but later on in life also in 'females', so that all birds become 

 potentially 'male' at the end of their life. 



Of the large number of oocytes only few ripen to make the enormous 

 follicles. After each follicle has burst it quickly regresses; there is no 

 'corpus luteum'. 



The egg is taken up by the ciliated and muscular funnel of the left 

 oviduct, and passes down a tube with circular and longitudinal muscles 

 and a glandular, ciliated mucosa. The albumen of the egg is produced 

 by long tubular glands, opening to the lumen. The oviduct has various 



