IX 



SEX 



89 



as due to the presence of an inhibitory factor which 

 prevents the full development of the black pigment 

 (cf. p. 6^^. Inhibitory factors appear to play a con- 

 siderable part in poultry, and another inhibitory 

 factor offers an interesting example of the sex-limited 

 type of inheritance which we have just discussed. 



The Silky breed of fowls is characterised among 

 other peculiarities by a remarkable abundance of 

 melanic pigment. The skin is dull black, while the 

 comb and wattles are 



Silky 



Brown Leghorn 



X 6 



? 



<^ 



'F, 



i i 6 





? ? 9---F. 



Fig. 19. 



of a deep purple colour, 

 contrasting sharply 

 with the white plumage 

 (PI. VI., 3). Dissection 

 shows that this black 

 pigment is widely 

 spread throughout the 

 body, being especially 

 marked in such mem- 

 branes as the mesen- 

 teries, the periosteum, 

 and the pia mater sur- 

 rounding the brain. It also occurs in the connective 

 tissues among the muscles. In the Brown Leghorn, 

 on the other hand, this pigment is not found. Recip- 

 rocal crosses between these two breeds gave a remark- 

 able difference in result. A cross between the Silky 

 hen and the Brown Leghorn cock produced F^ 

 birds, in which both sexes exhibited only traces of 

 the pigment. On casual observation thc\' might 

 have passed for unpigmented birds, for with the 

 exception of an occasional fleck of pigment their 

 skin, comb, and wattles were as clear as in the 



Scheme illustrating the result of crossing a 

 Silky hen with a Brown Leghorn cock. 

 Black sex signs denote deeply pigmented 

 birds, and light sex signs those without 



Cigmentation. '1 he light signs with a 

 lack dot in the centre denote birds with 

 a small amount of pigment. 



