230 



determines the sex, since the factor R is transmitted by the 

 male only to his female offspring. American writers have 

 correlated this fact with the discovery that in the male of 

 Drosophila there is one hétérochromosome, transmitted to only 

 half the spermatozoa, while in the female there are two, so that 

 one is transmitted to every egg. They assume that an egg fer- 

 tiHsed by a spermatozoon bearing a hétérochromosome becomes 

 a female, one by a spermatozoon lacking a hétérochromosome 

 becomes a male, and that sex-limited characters are borne 

 by the hétérochromosome, and thus transmitted by the male 

 only to his daughters. This view, though very simple, has 

 certain disadvantages : 



{a) It involves the assumption that in Moths and Birds, 

 which have sex-limited inheritance of the Abraxas type, the 

 sex is determined by the e^g, while in Diptera and Man, where 

 the Drosophila type is found, the spermatozoon determines the 

 sex. 



{b) There is also evidence that in Birds there is a single 

 hétérochromosome in the male, as in Drosophila ; and also that 

 the sex-ratio in Drosophila is strongly influenced, if not deter- 

 mined, by the female parent. 



(c) Finally, there is some evidence, not perhaps very satis- 

 factory, that in species in which there is absolute sex-limited 

 transmission in one sex, there may also be a slight degree of 

 sex-coupling in the other, e.g. that while the male Aa transmits 

 the factor A only to his daughters, the female Aa may produce 

 an excess of .4 ? and a ¿ gametes, and a deficiency of AS 

 and a ? . If this should prove to be so, it is impossible that 

 the sex in such a case can be determined only by the sperma- 

 tozoon. It is therefore of great importance that further cases 

 of sex-limited transmission should be discovered, in order that 

 these questions may be further elucidated, and it appears 

 probable that some insect which may be bred in quantity and 

 with rapidity is the form which is most likely to provide the 

 material which is required. 



Note added June 1913. 

 With regard to paragraph (b) above, Morgan has shown 

 {Science, vol. 36, Nov. 1912, p. 718) that the apparent influence 



