Apeil 8, 1921] 



SCIENCE 



341 



amount in the male than in the female. Now 

 if the only difference between the two sexes 

 lies in one ekromosome, it is conceivable that 

 one X might be dropped from each sex, with- 

 out disturbing the sex balance. Besides the 

 paired chromosomes which were alike in both 

 sexes, the female would now contain one X, 

 the male none. It is possible that this hypo- 

 thetical simpler condition actually preceded 

 the other, that the X chromosome really was 

 at first a structure developed in the egg and 

 handed on by this route from mother to 

 daughter as an exclusively female structure, 

 very much as extra-nuclear structures or 

 plastids (chromatophores) are handed on in 

 certain plants, or in the egg of the green 

 hydra, being never found in the male gamete. 

 It may be also that this condition is realized 

 in birds and moths, but of this more later. 

 'Now Wilson has shown, by comparative 

 studies of the sex-determining mechanism of 

 insects, that the single unpaired X of the male 

 is apt to acquire a mate which he calls T, of 

 unknown origin and function but certainly of 

 different nature from X. This is frequently 

 much smaller than its synaptic mate, X, but 

 in other cases is almost or quite as large as X, 

 so that the chromosome count shows the same 

 number of pairs in both sexes. Only compara- 

 tive studies, coupled with experiments in sex- 

 linked inheritance, show that throughout the 

 series there is an odd or single X in the male, 

 while in the female there is a pair of X's. The 

 T chromosome, which makes its appearance as 

 a mate of the odd X, is now a purely male 

 structure, counterpart of the hypothetical 

 original single X of the female, and Schmidt's 

 observations show that such a structure may 

 perfectly well be a vehicle of transmission in 

 heredity of characters which are the exclu- 

 sive possession of males. For males would now 

 form (as in Drosophila) two kinds of sperm, 

 differing only in one respect. One type con- 

 taining X would be female determining, the 

 other type containing Y would be male deter- 

 mining when respectively they fertilized the 

 single type of mature egg, which contained 

 one X. If, however, by non-disjunction in 



maturation an egg retained two X's and was 

 fertilized by a T sperm, it would of neces- 

 sity develop into a female (2X) (as is the 

 case in Drosophila, Bridges) which neverthe- 

 less would possess inherited characters pre- 

 viously possessed only by males, because of 

 the presence of the T. Eggs of this char- 

 acter might (as in Drosophila) produce four 

 types of gametes, viz., XX and Y, or X and 

 XY. Sperm also (as in Drosophila) might 

 be produced of the classes X, Y, and XY. A 

 great variety of possible combinations would 

 result, as Bridges has shown in cases of non- 

 disjunction in Drosophila. Of these various 

 combinations, two sets might give rise to 

 stable self -perpetuating systems of the Abraxas 

 type, viz., (1) female XX-Y, in which the two 

 X's are permanently united into a single body 

 which acts as the synaptic mate of Y, while 

 the male is Y-Y in formula; or (2) female 

 X-Y with an increased potency of X sufficient 

 to determine femaleness in single dose, male 

 Y-Y. It is true that in Drosophila Bridges 

 finds Y-Y zygotes non-viable but this is no 

 evidence that Y-Y zygotes would be non -viable 

 in all organisms. It is also true that he finds 

 that the non-disjoining X's may later sepa- 

 rate, but this would not preclude permanent 

 union of two X's in other organisms. 



On the other hand, it is conceivable that 

 the poultry type of sex-linked inheritance may 

 not have been derived from the Drosophila 

 type at all but from a simpler primitive con- 

 dition in which the female bore one X, the 

 male no X. If in a species of this type, a Y 

 chromosome appeared in the egg as the syn- 

 aptic mate of X, it would necessarily go ex- 

 clusively into those eggs which "were to be- 

 come males and would thus seem to be an 

 exclusive male jxissession even though it had 

 originated in a female. But the male which 

 had received Y from his mother would now 

 produce two types of sperm, Y and no-Y. An 

 egg transmitting Y, if fertilized by sperm also 

 Y would produce a Y-Y male, which might 

 prove to have greater survival value than the 

 male which contained no Y or only a single Y. 

 If this happened, the race would become per- 

 manently, female X-Y, male Y-Y, which in 



