riddle: control of sex ratio 329 



learned the following facts: (1) Generic crosses, when not per- 

 mitted to lay many eggs, produce mostly or only males. (2) 

 Such pairs, when made to lay many eggs (crowded reproduction) 

 produce males predominantly from their earlier, stronger eggs, 

 and predominantly or only females from the later eggs laid 

 under stress of overwork. (3) From the eggs of pure wild spe- 

 cies the first egg of the pair or clutch more often hatches a male ; 

 the second egg of the pair more often produces a female. 



These generic crosses, then, show practically full fertility and 

 exclude the possibility of accounting for the abnormal sex ratio 

 of either spring or autumn by any "assortative mating" of 

 germs, since the sperms by hypothesis are all alike, 5 and all of 

 the ova are fertilized and the resulting sex of all is known. 



From series of eggs produced by generic crosses, under "over- 

 work" it is therefore practicable to select a certain number of 

 eggs from near the first and from near the last of the season, and 

 have fair assurance that (in this type of mating) most if not all 

 of the earlier lot are prospectively male-producing, and most or 

 all of the later lot are female-producing eggs. It was this possi- 

 bility that enlisted my own first efforts in the study of sex. 

 And, since a single individual ovum or yolk of the pigeon is 

 large enough to permit a chemical analysis — our first study was 

 to determine whether possible chemical differences between the 

 male and female-producing ova exist and are discoverable. 

 The first analyses of the pigeon's ova were made in April, 1911, 

 and the work has been carried on continuously since that time. 

 Nearly 900 individual yolks have now been analyzed. Among 

 these are represented the eggs of several pure species, and of 

 many kinds of hybrids. The records for the chemical composi- 

 tion of the egg-yolks of a considerable number of individual fe- 

 males is now complete for five consecutive years. Altogether, 

 these studies, and the supplementary ones which developed out 

 of them or along with them, have brought to light a number of 

 facts which I can here only briefly sketch. 



Before considering the results of the analyses it may be well 

 to make clear the nature of a difference which appeared as soon 



5 It is certain that the ova are sexually dimorphic. 



