582 SCIENCE PROGRESS 



however, that the same principle will be found applicable to 

 these animals, and a similar expectation is created in case of 

 the rotifers and daphnids. 



These observations establish another and most important 

 point. It is a familiar fact that the parthenogenetic egg of the 

 aphid or phylloxeran (like that of the daphnid) whether male- 

 producing or female-producing, forms but one polar body, and 

 undergoes no general reduction of the chromosomes. Both 

 Morgan and von Baehr find that the males (produced partheno- 

 genetically) nevertheless differ from the females in the absence 

 of one X-element from the diploid groups ; and Morgan's work 

 shows, almost with certainty, that this element must be extruded 

 from the egg at the time the polar body forms. This remark- 

 able discovery throws a new light on the whole question of 

 sex-production in parthenogenesis, and indirectly demonstrates 

 that it is not fertilisation per se that determines sex but a 

 constellation of factors that may be established by other means 

 as well. 



We turn now to Baltzer's no less important work on the 

 sea-urchin egg, 1 which deals directly with the fertilisation- 

 stages. His results are based on the investigation not alone 

 of normally fertilised eggs but also of dispermic, merogonic 

 and hybrid fertilisations ; and the consistent results from these 

 various sources give the strongest grounds for their acceptance. 

 They show that in these animals it is the female that is the 

 heterogametic sex, while the male is homogametic. 



The fertilised eggs or zygotes and early cleavage-stages all 

 contain the same number of chromosomes (36), but are of two 

 types, in one of which all the chromosomes may be equally 

 paired, while in the other there is one pair that consists of 

 quite unlike members. These may provisionally be called 

 " F" and " G." Study of the gamete-nuclei (in the fertilisation- 

 stages) proves that F is always derived from the egg-nucleus, 

 and never appears in any of the sperm-nuclei. F is however 

 present in only half the egg-nuclei, its place being taken in 

 the other half by G; while all the sperm-nuclei alike contain, 

 in place of F a chromosome indistinguishable from G. Nonfj 

 of the eggs were reared to maturity, nor was the history of 

 maturation followed out. It nevertheless seems certain that 

 fertilised eggs containing F must be females (since F is confint-d 



1 Op. cit. 



