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SCIENCE 



[N. S. Vol. XXIX. No. 740 



females, we get no change in the character of 

 the results shown in the column headed " off- 

 spring." See Table II. That is, the facts 



agree with the hypothesis, Xi^2, no-X = d', 

 quite as well as with the Bateson-Doncaster 

 hypothesis. But if we apply Wilson's 

 XX := 2, X ^ c?, hypothesis to the case, the 

 expectations for crosses 3 and 4 will be exactly 

 interchanged; cross 3 should produce only L?s 

 and Gc?s, whereas cross 4 should produce all 

 four possible combinations. This fact is de- 

 cisive against the Wilson hypothesis and for 

 that of Doncaster, or for such a modification 

 of it as I have attempted to present. 



We may, it seems to me, summarize our 

 present knowledge of sex-inheritance under 

 one consistent scheme, somewhat as follows : 



1. Sex is not directly controlled by the en- 

 vironment, but is determined by internal 

 (gametic) factors. 



2. The determination of sex depends upon 

 the presence in the zygote of a factor or fac- 

 tors which are inherited in accordance with 

 Mendel's law. 



3. Femaleness, that is, the capacity to pro- 

 duce macrogametes (eggs) depends upon the 

 presence of some factor wanting in the male. 



4. The presence of this factor is in heredity 

 dominant over its absence. 



5. As regards the transmission of this factor 

 we can recognize two distinct categories of 

 eases : 



A. Femaleness is attained only when the 

 differential factor is doubly represented in the 

 individual. In such cases the female is a 

 homozygote (XX), and the egg invariably 

 transmits the differential factor. Sex deter- 

 mination then rests with the male parent, for 

 half the spermatozoa possess the differential 

 factor and half lack it. The female is a homo- 

 zygous dominant, not, as Correns supposed, 

 recessive; whereas the male is a heterozygous 

 dominant, pure recessives being unknown. 



B. Femaleness is attained whenever the dif- 

 ferential factor is present in one only of the 

 conjugating gametes which produce the indi- 

 vidual. The gamete which transmits the dif- 

 ferential factor is of course the macrogamete 

 (egg), since this factor is not possessed by the 

 male parent. The female is a heterozygous 

 dominant, the male a pure recessive; homo- 

 zygous dominants are unknown. 



The experimental proof for the existence of 

 these two categories of cases has been produced 

 for class A by Correns, and for class B by 

 Doncaster and others. Cytological evidence 

 which strongly supports the interpretation 

 given to class A has been produced by Mc- 

 Clung, Stevens, Morgan and especially by 

 Wilson. This evidence is fully corroborated 

 by the work of many others. Direct cytolog- 

 ical evidence for the existence of class B is 

 not known at present, but may confidently be 

 looked for. 



6. The hypothesis which I advanced in 1903, 

 that both sexes are in the same species sex- 

 heterozygotes, is not supported by the consid- 

 erable body of evidence since accumulated. 



If, as seems probable, the differential sex- 

 character has its cytological basis in the " X- 

 element," as Wilson designates it, it becomes 

 an interesting question, what is the cytological 

 basis of those numerous morphological char- 

 acters possessed by the male, but wanting in 

 the female. For it is a well-known fact that 

 such secondary sexual characters are in gen- 

 eral both more numerous and more striking 

 in the male than in the female. For this 

 reason the male has been called the " progres- 

 sive " sex, which takes on new or striking 

 characters, that may or may not later be 



