BELATING TO SECONDARY SEXUAL CHARACTERS. 97 



mits hen-feathering in the same way as does the male, and also by the 

 fact, as Darwin pointed out, that an old female Sebright whose ovaries 

 had degenerated developed not the hen-feathered plimaage of her own 

 cock, but cock-feathered plumage like that of most male poultry. 



13. The color of the Fi birds is shown in plate 2, figs. 1 and 2. In 

 general, the feathers are stippled, black and Ught yellow being the two 

 most conspicuous ingredients. Since hen-feathering dominates, the 

 dimorphism is absent, or at least is so slight as to not attract attention 

 — ^Uttle more, in fact, than in the Sebright race. The carriage of the male 

 is Uke that of the Sebright male. The Fi male and female are alike in 

 the direct cross and the reciprocal, or at least no conspicuous differ- 

 ence is found between the two classes of hens, indicating that no impor- 

 tant sex-linked factors are involved in the cross. 



14. The F2 birds show a great variety of color and pattern, but those 

 obtained can be approximately grouped iato 16 classes. The classes 

 are, however, admittedly not uniform, indicating minor factors not 

 here reckoned with. The classification of the hens is easiest; the Fa 

 hen-feathered males can then in many cases be referred to the proper 

 classes; the F2 cock-feathered males can not be accurately classified 

 with their corresponding hens, except in the case of those that resemble 

 the two Pi males, the Fi male, and those that castration experiments 

 of the hen-feathered males have shown to belong to certain hen types. 



15. Despite the admitted difficulties of classification, it is suggested 

 that three factor-pairs of differences will cover the main color classes 

 seen in the F2 and in the back-cross. One or two of these seem to be 

 incompletely dominant, since the Fi birds are not like either parent in 

 any single character, nor are they Uke the wild tj^pe in so far as this is 

 represented by the game. 



16. A histological examination of the testis of the male Sebright by 

 Boring and Morgan has shown that it contains cells like those present 

 in the ovary of all breeds of poultry. These cells are called luteal cells 

 by Pearl and Boring, from their resemblance to the cells of that name 

 found in the corpora lutea of mammals. In the mammals similar cells 

 are supposed to produce internal secretions that act as hormones. 

 Their function in the female bird is unknown, but the fact that after 

 the removal of the ovary the female develops the secondary sexual 

 plumage of the male suggests that some secretion from these cells 

 performs this function. Their occurrence in the male Sebright and their 

 complete absence, or paucity, in the males of other races supports 

 strongly the view that these cells are concerned with the suppression 

 of the secondary sexual plumage. 



17. While in mammals the interstitial cells have been supposed to 

 produce an internal secretion that causes the development of some of 

 the secondary sexual characters of the male, and the fuller elaboration 

 of others, in birds no such connection exists, if we except the case 



