riddle: control of sex ratio 321 



experiment decreased or suppressed the production, or hindered 

 the union, or modified the chromosomal constitution, of one of 

 the types of germ cell and left the other type normal and func- 

 tional? These possibilities for accounting for abnormal sex- 

 ratios certainly exist and they must be squarely met by decisive 

 experiment. The facts from this sphere which we shall most 

 need to bear in mind while examining our series of ratios are as 

 follows : 



1. Sperm and ovum are two kinds of sex cells in respect of 

 their origin from the two contrasted sexes; but, beyond this, it 

 is now clear that in some animals one and the same male pro- 

 duces spermatozoa of two kinds, and that these two kinds are 

 not equal in their prospective sex value. Still other animal 

 forms are known in which the female produces two kinds of 

 eggs, having opposite prospective sex value. Most groups of in- 

 sects and several mammals are known to produce two kinds of 

 sperm, while in moths (Lepidoptera) and birds the dimorphism 

 of the germs exists in the eggs. 



2. In forms which reproduce in part parthenogenetically — 

 such as the bee, gall fly, plant-louse, etc. — the sex is known to 

 bear certain relations to chromosome number, or to maturation 

 phenomena in the egg. 2 



3. In wide crosses among Echinoderms, Baltzer ('09) and 

 Tennent ('12) have shown that when the cross is made in one of 

 the two possible directions, some of the chromosomes proceeding 

 from the sperms are eliminated and do not take part in embryo- 

 formation. This type of chromosome behavior has been found, 

 however, only in crosses of very widely separated forms. 



Pure wild species of doves and pigeons have proved to be 

 almost ideal material for obtaining highly abnormal sex-ratios, 

 and for the analysis of the meaning or significance of the modi- 



2 Even if one fully concedes the "lethal factor" explanation which Morgan 

 (Science, N. 8. 36: 718. 1912) gives for a particular ratio (1 c? : 2 9 ) in Droso- 

 phila, a similar basis could not apply to most of the pigeon series, since here 

 every egg formed in the ovary can be accounted for, and in numerous series every 

 egg hatched. These same facts, together with the fact that it is the female 

 pigeon that is heterogametic, exclude the action of "assortative mating" as a 

 cause of the sex-ratios obtained in the pigeon. 



