THE DETERMINATION OF SEX 581 



above. On the other hand, some observers have sought to 

 discredit these results, including even so unmistakable a fact 

 as the differential division and the resulting dimorphism of 

 the spermatozoa. I think it may fairly be said, however, that 

 steadily accumulating evidence from many sources has now 

 placed the essential facts upon too firm and broad a basis to 

 admit of doubt. Scepticism has in some cases arisen from 

 insufficient observation or unsuitable technique, in others 

 from a confusing of different types or the examination of 

 unfavourable material. It has doubtless been due in some 

 cases also to the observation of forms in which two kinds of 

 spermatozoa cannot be distinguished, either because the female 

 is the heterogametic sex, or because the X- and Y-elements 

 are of equal size. 



Among: the most recent discoveries that have confirmed 

 and extended the general result none possess a greater interest 

 and importance than those of Morgan, von Baehr and Stevens 

 on the phylloxerans and aphids. These animals, like the 

 daphnids, rotifers and bees, are well adapted to afford a 

 crucial test of the matter ; for in them, as has long been 

 known, all .the fertilised eggs produce females. This seems 

 at first sight to present a formidable difficulty ; but the detailed 

 study of the phylloxerans and aphids converts this apparent 

 difficulty into a remarkable confirmation. Morgan 1 discovered 

 in Phylloxera, and von Baehr 2 independently in Aphis, that 

 half the secondary spermatocytes are much smaller than the 

 others and degenerate without producing spermatozoa. Further 

 — and this is the crucial point — those spermatocytes, and those 

 alone, which produce the functional spermatozoa receive the 

 accessory chromosome or X-element (in Aphis a single chromo- 

 some, in Phylloxera a double element). In these animals, 

 accordingly, only the X-class, or. female-producing spermatozoa 

 come to maturity, while the Y-class, or male-producing spermatozoa, 

 are abortive. All the fertilised eggs therefore produce females. 

 A long-standing riddle thus receives a simple and elegant 

 ■> solution, and one that is fully in accordance with the facts 

 9»een in the ordinary sexual forms. The bees and ants exhibit 

 ^lenomena that are evidently of somewhat similar type, though 

 the work of Meves and others still leaves us in the dark 

 regarding the details of the chromosomes. It seems probable, 

 1 Journ. Exp. Zool. 1909, 7. 2 Arch. f. Zellforsch. 1909., 3. 



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