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differential residing in two kinds of eggs and not in the 

 sperm. We may here recall that previous to our own 

 studies, breeding data obtained from other birds had in- 

 dicated that in the birds the sexually dimorphic germs 

 are borne by the female— or to use Mendelian terms, 

 that the female bird is heterozygous for sex. 



The second conclusion that must be drawn is that a 

 selective elimination of ova in the ovary does not occur 

 during "overwork," while mated to a mate of another 

 genus, nor otherwise, since the two kinds of ova are— 

 from their size relations— positively known to present 

 themselves under these, and under all the conditions 

 which have been studied. In other words, the generic 

 cross, which produces all or nearly all males in the spring, 

 and all or nearly all females in the autumn, is utilizing 

 in the spring a number of female-producing ova for the 

 production of males, and in the later season is utilizing 

 for the production of females ova one half of which had 

 initial inclinations for the production of males. Note 

 too that the evidence for the continued production during 

 the season of ova of two kinds as regards sex does not 

 rest alone on our knowledge of the dimorphic ova. For, 

 from breeding data we learn that if the same female 

 which threw all males in the spring and all females in the 

 autumn, had been mated to one of her own species, then 

 both males and females would certainly have appeared 

 at all seasons, and largely or wholly in relation to the 

 order of the eggs of the clutch, with but slighter effects 

 of season to be noted. If the overwork were extreme, a 

 predominance of females in late autumn might be ex- 

 pected ; but in the earlier season the sexes would surely 

 be found in nearly equal numbers. Several of the cor- 

 relations soon to be mentioned, moreover, further attest 

 that ova of two grades— in respect to sex— are produced 

 throughout the year. 



The data thus far examined exclude the possibilities 

 of accounting for the observed sex-ratios of the generic 

 cross on the basis of a selective action of the sperm, or 

 of a selective elimination of ova in the ovary. What 

 Hght do these data shed on the possibility of accounting 



