686 THE PHYSIOLOGY OF REPRODUCTION 



male or female characteristics is conditioned by the level or rate 

 of the metabolic processes. Large size of yolk, a low percentage of 

 water in the yolk, a high percentage of stored material and a high 

 total of stored energy, and an exhausted physical condition or 

 advanced age in the mother are all associated with a low level of 

 metabolism and with the female sex. 



The work on pigeons and doves was begun by the late Professor 

 Whitman, and continued after his death by Riddle, who confirmed 

 and extended Whitman's original results. They showed that with 

 crosses the percentage of males increases with the width of the cross 

 until sterility is reached. Since males are characterised by a more 

 active metabolism, the results are in agreement with the commonly 

 observed superior vigour of hybrids. By overworking the birds, 

 that is to say, by not allowing them to nest their own eggs and 

 forcing them to continue laying eggs in rapid succession, the first 

 several pairs of eggs laid in the spring produced all or nearly all 

 males, whereas the last several pairs of eggs laid in the autumn gave 

 rise almost or quite exclusively to females, while both sexes were 

 produced in the summer or transition period. These results are in 

 correlation with the fact that the developmental energy is greatest 

 early in the season, and then declines, so that Whitman found that 

 at the extreme end of the season the eggs were unable to hatch 

 at all. 



The possibility of assortative mating and differential death-rates 

 are fully considered and rejected. Thus hens which when crossed 

 throw all males will, when mated to birds of their own kind, produce 

 both sexes equally. Moreover, the eggs are of two kinds, large and 

 small, and these in the wild pigeon are normally associated with 

 females and males respectively, but in the crossing experiments the 

 birds utilise " female-producing " ova for the formation of males, and 

 vice versa, the eggs thus changing their original sexes. 



Furthermore, Riddle finds grades of sexual individuals, some 

 females being more masculine than others, and some males coming 

 to resemble females, and if ovarian extracts are injected into the 

 more masculine females, and extracts of testis into the more feminine 

 ones, the sex-behaviour of the birds may be very largely reversed, 

 but the effect is not permanent. 



Lastly, the sex of the birds could not only be changed ; it could 

 also be accentuated. The hens hatched from the second clutch laid 

 in the autumn by overworked pigeons were found to be even more 

 pronouncedly female than normal birds, inasmuch as the right ovary 

 which usually atrophies, in these circumstances persists. 



Needless to say, Riddle rejects the chromosome theory as an 

 explanation of how sex is determined, and in support of his con- 



