No. 595] 



SEX CONTROL IN PIGEONS 



391 



in the pure wild species with which he worked, males 

 predominate in hatches from first eggs of the clutch, and 

 females predominate in hatches from second eggs of the 

 clutch, it became evident that the male-producing yolk 

 is smaller— both in relation to season, and to egg of 

 clutch, than is the female-producing yolk. Correspond- 

 ing to the fact (commonly obtained from matings of in- 

 dividuals of the same species) that two males or two 

 females may sometimes arise from the same clutch, we 

 have found that a similar number of pairs of yolks of 

 these forms are equal in size ; and too that such pairs may 

 be either large or small. The charts just referred to may 

 be consulted in this connection. We have previously 

 noted (1911, 1912) that in eggs laid by hybrids neither sex 

 nor yolk-size bears the above described relations to the 

 order of eggs in the clutch. 



Still a third situation has yielded positive evidence 

 that the smaller yolks are male-producing and the larger 

 yolks female-producing— namely that in respect to age. 

 It has already been mentioned that Whitman learned 

 that the females which were "overworked" tended, when 

 older, to begin the production of females at earlier and 

 earlier stages of the season. Now a comparison of the 

 size of yolks derived from younger and from older birds 

 has conclusively shown smaller eggs for mature but 

 younger birds, as compared with the old birds (see Chart 

 2). In scores of individual cases the yolk-size has now 

 been followed from youth, and comparative youth, to 

 old age. 



In even a fourth situation it has been possible to test 

 the relation of yolk-size to sex. Breeding data show that 

 from the very first egg in life, and the very first egg pro- 

 duced after a long period of rest or inactivity, more fre- 

 quently produce a female than do the first eggs of succeed- 

 ing pairs, or clutches. Our studies on the size of such 

 yolks show a wholly similar reversal of order of size of 

 the two eggs of the very first clutches ; the size reversals 

 here being more frequent than in the succeeding clutches 

 (see Chart 4). 



