110 



Mr. L. Doncaster. 



[Sept. 21, 



shown that those which degenerate lack the heterochromosome, i.e. those 

 which we have represented by ©. The spermatozoa are thus all ^ -bearing, 

 but since the sexual female lacks the ^ element, all the eggs are ? , and the 

 resulting offspring (zygotes) are females with constitution ? J' . Morgan 

 assumes that the spermatozoon determines the sex, but this involves the 

 belief that the male Aphid contains a ? determinant, and since we know that 

 the parthenogenetic female contains the factor (since males are produced 

 from it), this would involve the complex assumption of alternative sex- 

 dominance. If my hypothesis is correct, it could be tested in Aphis by 

 observing whether, in the single polar division by which a male egg is pro- 

 duced, one complete heterochromosome is extruded, thus leaving only one 

 heterochromosome in the egg instead of the two which are characteristic of 

 the parthenogenetic females. 



In this discussion I have not attempted to deal with the case of the saw- 

 flies (Tenthredinidse), in which facultative parthenogenesis occurs. In former 

 papers* I have discussed this question, but subsequent observations have led 

 me to believe that the work requires extension and revision, and since this is 

 still in progress, I prefer to leave any disctlssion until the results are clear. 



A final word should be given to the recent hypothesis of Wilsonf and 

 Castle,+ that the sex-determinants are not - and ? -bearing resi)ectively, but 

 that only one kind of determinant exists, and that the female contains one 

 more such determinant than the male. Castle supposes that in some species 

 the female contains two such determinants, the male one ; in other species, 

 the female one and the male none. The existence of two kinds of partheno- 

 genetic females in Ncuroterus cannot be explained on either assumption, but 

 it is conceivable that both kinds of parthenogenetic female contain one sex- 

 factor and the male none. The difl'erence between the arrhenotokous and 

 thelytokous female would then be that the eggs of the former contained a 

 mechanism for expelling the sex-factor, while the thelytokous egg would 

 retain it. The extranuclear body found in half the spermatids nught be 

 responsible for this difference. This, however, ducss not seem very probable. 

 The hypothesis, also, cannot explain the almost invariul)le excess of aflectcd 

 males in cases of sex-limited inheritance, such as colour-blindness, which is 

 accounted for if we assume that the factor for the disease is more often borne 

 by than liy ? ova. In general, the hypotliosis doi^s not secnu to bring 

 together such a wide range of phenomena as tliat suggested above. 



* 'Q.J.M.S.,' vol. 49, 1906, p. 501 and vol. 51, 1!)07, p. 101. 

 t 'Science,' January, 1909, vol. 29, \>. r,3. 



I 'Science,' March, 1909, vol. 29, p. 395. See also Morgan, 'Journ. Exp. Zoo., 

 Sept., 1909, 1). 332. 



