I go Summary of Evidence as to Sex [ch. 



the femaleness. It is of especial interest to know that in 

 this case the accessory chromosome goes through a stage 

 in which it is partially divided into two, though after this 

 partial division the two parts reunite and then the whole 

 passes into one of the daughter-cells. The point that 

 remains obscure is the nature of the cell-division by which 

 the males are produced. For at one stage in the cycle 

 males are of course born parthenogenetically from the 

 females, and it would be of the greatest importance to 

 know something of the cytological process by which the ova 

 are destined to become males — or male-bearing females 

 if there are two lines (as in the Aphides studied by Miss 

 Stevens, 256, 260)^. Many other very interesting ques- 

 tions arise in connection with this part of the evidence, but 

 further discussion must be postponed pending the accumu- 

 lation of further evidence. 



Summary of Experimental Evidence as to the Heredity 



of Sex. 



In a full discussion of the problem of sex-inheritance 

 many other kinds of fact would need to be considered. To 

 these no reference can now be made. All that I have 

 attempted is to provide a sketch of the new evidence on the 

 problem which Mendelian experiments contribute. The 

 main conclusion to which several quite distinct lines of 

 inquiry unmistakably point, is that in the two Vertebrates 

 and in the Currant moth the female is a sex-heterozygote, 

 with femaleness dominant. The female is a hybrid, "female- 

 male," while the male is pure male, or '' male-male." The 

 eggs of the female are thus females and males respectivelyt, 

 while the spermatozoa are all male. In other words, the 

 female contains a factor which makes her female, but the 

 male is male because he is without this factor. The 

 phenomena can, as has been shown, with certain exceptions, 

 be represented symbolically by a system based on this 

 conclusion. The exceptions are real, but they are manifestly 

 exceptional, and for the present we may be content to deal 

 with the main course of the descent. It may conceivably 



* These papers contain an account of the beginning of a very valuable 

 experiment on the descent of colour in Aphis and its relation to sex. 

 t As those of Difiophilus apatris are. 



