104 



Mr. L. Doncaster. 



[Sept. 21, 



The egg which is to undergo maturation may be regarded as containing 

 both male and female determinants ; the female determinant is removed in 

 the polar mitosis and the egg remains male. The eggs which will undergo 

 no reduction may be considered as containing only a female determinant, and 

 this causes the egg to develop into a female. 



If, as suggested, the ovarian eggs of some parthenogenetic females contain 

 both male and female determinants, while those of others contain a female 

 determinant but not a male, the difference between the two kinds of eggs 

 must be caused by a difference of constitution in the females which lay 

 them. Both kinds of female are produced from fertilised eggs, and the 

 difference in constitution between them may be due to the existence of 

 two kinds of spermatozoa in the males of the previous generation. The 

 existence of spermatozoa of two kinds is indicated by a number of facts 

 which will be further discussed below, but here it will suffice to refer to 

 the evidence in the Hymenoptera and other parthenogenetic insects. In 

 Neuroterus itself I have mentioned above that half the spermatids receive 

 an extranuclear body which is not found in the other half, and it is 

 conceivable that this is connected with' the difference under discussion. 

 But the fact that in the Bee half the spermatid nuclei degenerate, while in 

 other related Hymenoptera all develop into spermatozoa, suggests that two 

 kinds of spermatozoa may exist. And a similar condition has been described 

 by Morgan* in Phylloxera, and confirmed in a fuller paper by von Baehrf in 

 Aphis. They find that there is an odd number of chromosomes in the somatic 

 nuclei of the male, and at the first division of the spermatocytes one 

 chromosome goes over undivided to one end of the spindle, so that one 

 daughter nucleus receives a " heterochromosome," while the other does not. 

 In the second division the heterochromosome divides equally, but only those 

 secondary spermatocytes which contain it develop further, the others 

 degenerating. Thus, as in the Bee, half the daughter-cells of the primary 

 spermatocytes atrophy, and since all fertilised eggs become females, Morgan 

 supposes that tlie heterochromosome, which is contained in the functional 

 spermatids, bears the female determinant. A different explanation of the 

 facts will be offered below, but the facts themselves clearly indicate the 

 existence of two kinds of spermatids, one of which, as in Neuroterus, contains 

 a body lacking in the other. We may suppose, then, that in Neuroterus, one 

 kind of H])ermatoz()on, fertilising an egg of the sunnner generation, gives 

 rise to female-producing (th(;lyt()kou,s) female, tlie otlier kind to a male- 



* ' I'roc. Soc. Kxper. J'liol. and Mod.,' ]!J08, vol. 5, p. 5G. (Full ])ai)ei, ' .Journ. Exp. 

 Zoo.,' vol. 7, 1909, p. 239.) 



t 'Zool. An/.eiger,* vol. 33, 1908, No. 15. 



