CHROMOSOMES AND SEX-DETERMINATION 399 



an unequivocal result. The method adopted was to pair 

 females belonging to all-female families, allow them to lay 

 eggs as far as possible mider observation, and to preserve the 

 first 50 or 100 eggs at an age (about two hours) when the 

 maturation-divisions would be in progress. The moths were 

 then allowed to continue laying, the eggs counted, and reared 

 either to the imago or to larvae in which the sex could easily 

 be determined by dissection. 8ome of these families produced 

 both sexes, others either females only or females in great 

 excess. The preserved eggs of families which proved all-female 

 were then sectioned, and counts made of the chromosomes in 

 the polar division-spindles. 



By the summer of 1915 I had already enough material to 

 show that the hypothesis put forward in the 1914 paper was 

 almost certainly incorrect, and since work in connexion with 

 the war prevented the immediate continuation of the investiga- 

 tion I published a preliminary note in a letter to * Nature ' 

 (June 10, 1915) in which I wrote as follows : ' I have now 

 examined the eggs of several such families [i. e. all-female 

 families], and find, contrary to expectation, that the equatorial 

 plate of the inner spindle contains twenty-eight chromosomes 

 about as frequently as twenty-seven. The new material con- 

 firms the observation that twenty-seven occur in one spindle 

 and twenty-eight in the other, but it seems to make it certain 

 that the presence of twenty-eight chromosomes in the inner 

 spindle does not necessarily cause the production of a male — 

 at least in the strain which produces all-female families. A 

 possible explanation of the anomaly is that in all-female 

 families a chromosome is eliminated at a later stage, but at 

 present I have no direct evidence for this.' 



From that time to the summer of 1919 the work was inter- 

 rupted, but enough material had been collected to provide 

 the required observations, and examination of the sections 

 confirms the account shortly given in the letter quoted. There 

 are two questions at issue : (1) whether the all-female families 

 are so because all the fertilized eggs are truly female, or whether 

 they arise through non- viability of male zygotes ; (2) if all 



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