138 EAYMOND PEARL 



try (Pearl 22). Special pens have been devoted to this experi- 

 ment from the beginning and the birds have not been moved 

 about from them except for short periods for purposes of clean- 

 ing. It is scarcely necessary to add that control and treated 

 birds have been fed and managed in the same way, every effort 

 being made to see that the only differential factor in the manage- 

 ment of the two sets of birds was the inhalation treatment. 



C. Foundation stock used in the experiments 



In table 2 is given a complete conspectus of the foundation 

 stock used. 



From this table it will be seen, in the first place, that the 

 birds of the Barred Plymouth Rock breed used were all females, 

 and the birds of the Black Hamburg race used were males. 

 The cross was made in this way since the barred color pattern of 

 the Barred Rock is known to be a sex linked character for which 

 the female is heterozygous (Goodale, 8, Pearl and Surface 28 

 and others). By making the cross in this way additional pos- 

 sibilities in the variety of character combination are opened. 

 In the second place, it is evident from table 2 that the birds 

 were so chosen that each treated bird had from one to five or 

 six untreated sisters to serve as controls. This is believed to be 

 an important critical element in the conduct of the experiments. 



Another reason for choosing Barred Rock females for the 

 experiments is that they are a docile breed, easily tamed and 

 not nervous or easily frightened when properly handled. These 

 are important factors in the critical conduct of an experiment 

 of this sort. It is easily within the bounds of possibihty, in 

 fact experience would lead one to regard it as even probable, 

 that quite erroneous conclusions might be reached in experi- 

 mental work of this sort if females of some very nervous, easily 

 excited and flighty breed, as the White Leghorn for example, 

 were used. Daily tanking might very well put then off their 

 laying to a marked degree, even though the tank contained 

 nothing whatever but just plain air. Meantime the untanked 

 controls would be laying normally, and one would be almost sure 



