CYTOLOGICAL PROBLEMS 73 



the females are quantitatively greater in some substance than 

 the males : " femaleness is due to maleness plus something 

 else." It will readily be seen that natural parthenogenesis 

 accords with this view. In all cases of male-producing eggs 

 the chromosomes are halved in number (Hymenoptera, Cyni- 

 pidae, Rotifera), or an X chromosome is omitted (Aphidae) ; 

 while in all female-producing eggs the chromosome number is 

 complete. How then do the few observations on the sex of 

 artificially produced parthenogenetic animals fit this hypo- 

 thesis ? 



Take, first, the case of the sea-urchin. It is found that 

 during and after treatment the nucleus and cytoplasm of the 

 eggs undergo changes, comparable with those occurring at 

 fertilisation. Asters appear at one of the nuclear poles and the 

 centrosomes divide, their halves diverge, and develop a typical 

 spindle between them. During these changes the nucleus 

 enters the prophase of division. The chromosomes are formed, 

 and, on the disappearance of the membrane, take up an 

 equatorial position on the spindle, separate, and pass to each 

 pole. Thus, each daughter nucleus has the half number of 

 chromosomes (haploid), and this reduced number is retained 

 at least through the early succeeding divisions. Though this 

 was at first denied, later investigations have shown beyond 

 doubt that it obtains. Thus, the parthenogenetically produced 

 sea-urchin is, in respect of its nuclei, comparable with the males 

 of the Hymenoptera. If, then, further work shows that only 

 males are produced from treated sea-urchin eggs, the explanation 

 offered for the bee holds equally well here. If, on the other 

 hand, it is found that female sea-urchins can also develop, it 

 may be explained on one of the lines I suggested in an earlier 

 paper for the case quoted by Jack and mentioned above. 



(a) There may be two maturation divisions, but both 

 equatorial, as in some of the sawflies, in which case there would 

 be no chromosome reduction. 



(b) There may be but one maturation division, which is 

 not reductive. 



(c) There may be non-disjunction of the sex-chromosomes, 

 if such exist. 



This phenomenon of non-disjunction was suggested by 

 Bridge, as a result of work on Drosophila. He found that 

 occasionally the sex-chromosomes of the egg stuck together 

 at the maturation divisions, both being extruded with the 

 polar body, or both remaining in the female pro-nucleus, in 

 which case the resulting animal would be a female. 



Either of these three possibilities would explain the inter- 

 esting case of female production in the bee or partheno- 

 genetically produced female sea-urchin. Or, finally, if in 



