Januaet 8, 1909] 



SCIENCE 



61 



parthenogenetic forms, whicli have liitherto 

 seemed to constitute a serious difficulty. 

 Some years ago Meves discovered that in 

 the bee half the spermatocytes are very 

 small and degenerate without forming func- 

 tional spermatozoa, and Meves compared 

 these to polar bodies, but seems not to have 

 suspected their significance in relation to 

 sex-production. Morgan discovered in 

 Phylloxera and von Baehr independently 

 in Aphis saliceti, that a similar degenera- 

 tion of half the spermatocytes takes place, 

 and further, that these are the ones that 

 fail to receive the ' ' accessory chromosome ' ' 

 (i. e., the X-element). Functional sperma- 

 tozoa are produced only from those sperma- 

 tocytes into which the accessory chromo- 

 some passes, and these obviously correspond 

 exactly to the female-producing class in the 

 ordinary case. These observations have 

 since been extended by Miss Stevens to a 

 considerable number of species of aphids. 

 A complete explanation is thus given of 

 the fact, which has long been a puzzle, that 

 in these animals all the fertilized eggs pro- 

 duce females. Not less interesting is the 

 discovery by Morgan and von Baehr that 

 in both the forms in question the males, 

 though produced from the females strictly 

 by parthenogenesis, have one chromosome 

 fewer than the females. The male-pro- 

 ducing egg must therefore eliminate one 

 chromosome, and this, we can not doubt, 

 is the X-element. "What has hitherto 

 seemed to be a stumbling-block in the way 

 of the general conclusion is thus seen to be 

 in reality a remarkable confirmation. 



To what extent these conclusions, based 

 upon the study of the arthropods, will be 

 found to hold true for other organisms re- 

 mains to be seen. The experimental re- 

 sults of Correns on the fiowering plants, 

 which harmonize completely with the cyto- 

 logical results on the insects, certainly 

 seem to give good reason to expect that 

 the general principle involved will be 



found to hold true of a large series of 

 forms. 



IV. SEXUAL PKEDESTINATION AND HEREDITY 



Deferring for the moment the question 

 of the sex-ratios, let us now attack the 

 most difficult but perhaps most interesting 

 part of our inquiry, which concerns the 

 nature of the sexual predestination and its 

 relation to the phenomena of heredity in 

 general.^ In the air-breathing arthropods, 

 as has been seen, a dual sexual predestina- 

 tion of the spermatozoa is clearly seen. 

 Does such dual predestination exist also in 

 case of the sexual eggs ? Could we rely on 

 the cytological evidence alone we should 

 unhesitatingly say, no; for it is clear that 

 all the mature eggs are cytologieally alike. 

 Moreover, in the aphids and daphnids and 

 rotifers the sexual eggs are all alike 

 destined to produce females ; and although 

 it is possible that a male-producing class 

 degenerates (like the corresponding class 

 of spermatozoa), there is no evidence of 

 this. The facts here evidently suggest 



' Here again caution in our use of terms 13 

 necessary. Obviously in the cases we have been 

 considering the spermatozoa are, in a purely 

 descriptive sense, predestined as male-producing 

 and female-producing. But it by no means fol- 

 lows that they are predetermined as male and 

 female or even that they are male-determining 

 and female-determining. Sexual predetermination 

 and sexual predestination must not be confused, 

 as is clearly shown by Corren's discovery that the 

 pollen grains . of diecious flowering plants are 

 prospectively predestined as male-producing and 

 female-producing, though their immediate pro- 

 ducts (the rudimentary prothallia) are all males. 

 It would seem that there are here two kinds of 

 males, which give rise, respectively, to male-pro- 

 ducing and to female-producing gametes. Clearly, 

 the definitive determination of maleness or female- 

 ness only occurs when all the factors necessary to 

 their production have been brought together. This 

 may be effected before fertilization ( " progamic 

 determination" of Haecker), but may also first 

 ensue upon union of the gametes ("syngamic de- 

 termination ") . 



