584 SCIENCE PROGRESS 



Some Theoretical Aspects of Sex-production 



Thus far we have held closely to the observed facts, leaving 

 aside all matters of theory ; for the conclusion that the two 

 classes of gametes in the heterogametic sex are respectively 

 male-producing and female-producing is not in any sense a 

 theoretical construction, but a direct and apparently unavoidable 

 conclusion from accurately known data. Now, however, we 

 may be allowed to consider the theoretical aspect of the problem. 

 In making some inquiry into the causal relation between sex 

 and the chromosomes, I wish not to appear as a special pleader 

 for any particular theory. We still know too little of the 

 physiological meaning of the chromosomes to commit ourselves 

 unreservedly to any fixed theory or interpretation regarding 

 the part that they play in sex-production. My intention is only 

 to analyse briefly the possibilities suggested by the facts. If 

 certain of these possibilities are characterised as more probable 

 than others, it is with entire readiness to accept a different view 

 as soon as new facts may change the aspect of the problem. 



The obvious view is that the chromosomes — specifically the 

 X-chromosomes — are actual sex-determining factors ; that the 

 egg becomes male or female according as it is fertilised by one 

 kind of spermatozoon or the other ; and that it is purely a 

 matter of chance which of these kinds enters the egg. I incline 

 to believe that this obvious view is the correct one, though it 

 requires some qualification. It is impossible to admit that the 

 definite relation between sex and the chromosomes can be a 

 merely casual one that is devoid of all significance in sex- 

 production. The most that can be urged against the obvious 

 view is that the chromosome combination may be not the cause 

 of sex but one of its results. There are indeed some facts that 

 may, if we choose, be interpreted in this sense. For instance, 

 it is possible that in the differential division, which s-nucieus, 

 the X-class from the Y, the two poles of the dividing ce ** .-— ~ 

 be already predestined as male-producing and female-producing, 

 irrespective of the chromosomes — i.e. that it is not a matter of 

 chance to which pole the X-chromosome wanders. We find, 

 it is true, no hint of such predestination in the ordinary forms 

 of spermatogenesis ; but in the phylloxerans and aphids, as 

 Morgan and von Baehr have shown, the differential division 

 is unequal, and the X-chromosome always passes into the larger 



