252 GENETICS AND EUGENICS 



To recapitulate, we have in fowls this relationship of 

 plumage and other secondary sex characters to the gonads 

 or their secretions. Fowls of both sexes will develop the 

 same plumage characters, viz., the full plumage of normal 

 males, if no secretions interfere. In females such an inhibit- 

 ing secretion is normally produced by luteal cells present in 

 the ovary, and in hen-feathered males luteal cells in the 

 testis produce a similar secretion. If luteal cells are intro- 

 duced into castrated males (in transplanted ovaries) the 

 birds become hen-feathered. Likewise if the luteal cells 

 are removed (with the ovary) from a female, she becomes 

 "cock-feathered." If the luteal cells are removed (with the 

 testes) from a hen-feathered cock, he becomes cock-feathered. 

 Hence "hen-feathering" in either sex is due to the secretion 

 of luteal cells, not to the sex-cells proper. But the developed 

 condition of comb and wattles normally seen in males is due 

 to a different secretion formed by the testis. For this con- 

 dition disappears in castrated males and is not attained in 

 feminized males into which ovaries have been introduced. ^ 



In male sheep a secretion of the testis seems to act as a 

 stimulant to horn development, for male sheep regularly have 

 larger horns than females (Fig. 97) and in some breeds, for 

 example the merino, males only have horns. (See Figs. 101 

 and 103.) Early castration of the male in such breeds results 

 in hornlessness. 



Finally Lillie (1916) has shown that in cattle hormones 

 in the blood of the developing male, if allowed to enter the 

 circulation of the developing female, so interfere with the 

 growth of the ovary as to render its possessor sterile. This 

 is the explanation of the "free martin," a sterile female calf 

 born as a twin to a male calf. The twins in this case begin 

 their development, each from a separate fertilized egg, but 

 become later so closely crowded together in the uterus of the 

 mother that their foetal blood vessels unite, allowing the 

 blood from one embryo to pass freely over into the other. 

 A sterilizing influence on the female results, the ova in the 

 body of the female embryo failing to grow, but no reciprocal 



