72 SCIENCE PROGRESS 



sexes may arise ; whereas, in the sea-urchin, although only 

 the one sex (male) has as yet been obtained, further work may 

 show that both males and females can develop. Such results 

 can, however, be paralleled in natural parthenogenesis where 

 it is easy to find examples of dual sex-production, or the 

 development of only one sex, male or female. Thus, many 

 species of the Aphidae, Cynipidae, and Cladocera produce both 

 sexes. Among the bees and wasps the unfertilised eggs give 

 rise to males, while among some species of the Tenthredinidae 

 (Sawflies) only females develop from such eggs. These pheno- 

 mena easily fall into line with the sex-chromosome hypothesis 

 if the cytological basis is considered. 



In the Rotifera and Cynipidse, where both sexes are produced 

 from parthenogenetic eggs, those developing into males undergo 

 two maturation divisions and the chromosomes are halved in 

 number. The female-producing eggs, on the other hand, have 

 but one maturation division, which is equational, the chromo- 

 some number remaining unchanged. 



In the Hymenoptera (ants, bees, and wasps), as mentioned 

 above, males are produced from the parthenogenetic eggs, 

 having the reduced chromosome number, as in the Rotifera, 

 whereas females hatch from fertilised eggs, containing the full 

 complement of chromosomes. There is an interesting case 

 described by Jack,^ where, on rare occasions, worker bees 

 produce females from unfertilised eggs, but until their cytology 

 is known discussion is profitless. 



The Aphids offer many interesting and perplexing problems 

 in sex-determination, but a few cases have been investigated 

 by Morgan, Stevens, and others. Here, again, both sexes 

 arise from parthenogenetic eggs, differing, however, in their 

 chromosome complex. Both types undergo one maturation 

 division, but in the male-producing eggs one of the X chromo- 

 somes is eliminated, while in the female-producing ones both 

 X chromosomes are retained. It is impossible to deal here 

 with fertilisation and the associated changes in the chromosome 

 complex in the above species ; this question is, however, 

 discussed in an earlier paper.* Finally, in those species whose 

 parthenogenetic eggs give rise to females only it is found that, 

 whatever may be the number of maturation divisions, the 

 chromosomes are not reduced. 



It will be remembered that in the previous article in this 

 Journal the theory of sex-determination was favoured which 

 ascribed sex as dependent on the quantity of sex-determining 

 substances contained in the egg. In the majority of animals 



' Jack, R. W., Trans. Entom. Soc, 1916. 



* Cutler, D, W,, Mem. and Proc, Manchester Lit. and Phil. Soc, 62, 1917- 

 18. 



