1909.] 



Gametogenesis of the Gall-Fly, etc. 



109 



as they are in the Bee, and these purely female individuals might develop into 

 workers which would not lay eggs. It is generally considered that when 

 worker ants lay eggs, these always yield males (as in the Wasp and Bee), but 

 Eeichenbach* describes the case of a nest in which the workers produced 

 females except at the season when males normally occur, and then males 

 were produced. If some of the workers were of the ? O constitution, others 

 ? c?, this might thus be explained. The case clearly requires further 

 investigation. 



The hypothesis of sex-determination outlined above is doubtless highly 

 speculative, but it has the advantage that it brings into line several sets of 

 facts which have hitherto seemed irreconcilable, viz., the results obtained by 

 breeding such cases as Abrast:as, and also such cases as colour blindness, the 

 cytological observations of Wilson and others on heterochromosomes, and 

 the peculiar behaviour of the Bee and other Hymenoptera which have 

 facultative parthenogenesis. 



The fact that in NeurotervA the parthenogenetic generation is separated 

 from the bisexual one, and that some parthenogenetic females give rise only 

 to males, others only to females, supplies the clue, for it is clear that 

 some difference must exist in the constitution of the spring-brood females to 

 account for the difference in sex in the offspring of different individuals. 

 That this factor is introduced by the male parent seems probable from the 

 known fact that in so many cases spermatozoa of two kinds are produced. 



In the Aphides a variety of conditions occurs in different cases, but here 

 also the suggested explanation holds good. In all species fertilised eggs 

 yield females, and these produce a varying number of parthogenetic genera- 

 tions. All these must be regarded as of constitution ^ S • In some, the 

 later parthenogenetic females give rise to both sexual females and males from 

 the same individual ;t in these it may be assumed that in the eggs which 

 develop into sexual females the ^ determinant is removed ; from those which 

 yield males the ? determinant. In other cases the parthenogenetic females 

 give rise to " sexuparee," some of which produce parthenogenetically only 

 males, others sexual females. In these, the female-producing sexuparse may 

 have lost the S determinant, while the male-producing sexuparai contain 

 both and ? , but the ? determinant is removed with the polar body of 

 the egg which yields a male. Morgan and von Baehr have shown in a species 

 of Phylloxera and in Aphis saliceti that half the secondary spermatocytes 

 degenerate, as do half the spermatids in the Bee, and in this case they have 



* ' Biol. Centralblatt,' vol. 22, p. 461. 



t N. M. Stevens, 'Carnegie Inst. Public, Washington,' No. 51, 1906 ; also von Baehr, 

 loc. cit. 



VOL. LXXXII. — B. K 



