GERM-CELL HISTORY IN THE BROOK LAMPREY 101 



Whitman and, since his death, the experiments have been con- 

 tinued by Doctor Riddle. WTiitman found that, if certain dis- 

 tantly related pigeons were mated, for example, individuals of 

 different families, only male offspring resulted. If matings were 

 made of individuals not so distinctly related, as, for example, 

 between different genera, and to this was added the element of 

 overwork in reproduction, males only were produced in the early 

 part of the season and females only in the later part of the season. 

 He also observed that at the transition period during the summer 

 some pairs of eggs produced males and females, the first usually 

 male and the second female. It was noticed, further that toward 

 the end of the season the eggs w^ere not quite able to hatch, and 

 produced embryos of fewer and fewer days' development. This 

 led ^ATiitman to conclude that the developmental energy is 

 greatest in the male-producing season. 



Riddle, in a long series of experiments, has been able to verify 

 the results obtained by Professor Whitman. He has also dis- 

 covered many more facts which tend to show that in pigeons 

 there is a reversal of sex, and that under certain conditions male 

 offspring are hatched from normally female-producing germ cells, 

 and vice versa. 



In birds there should be, according to evidence obtained from 

 experimental breeding, two kinds of eggs; one maleproducing the 

 other femaleproducing. These two kinds should normally be 

 produced in equal numbers. Riddle does not deny the exist- 

 ence of a chromosomal difference in the eggs of birds. He 

 admits that it has been definitely shown that in some species, at 

 least, when bred under stable conditions, certain chromosomes 

 are associated with sex; but he denies that the sole cause of sex 

 lies in the sex chromosome and that sex is definitely fixed and 

 non-reversible from the very beginning of development. Data 

 collected, he says, ''strongly indicates that the basis of sex is a 

 fluid, reversible process; that the basis of adult sexual difference 

 is a quantitative rather than a qualitative thing." 



In pigeons, therefore, it has been shown that eggs which nor- 

 mally develop into males or into females can have their develop- 

 mental energy so changed by the introduction of spermatozoa 



