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SCIENCE 



[N. S. Vol. XXV. No. 63G 



give a true explanation in certain cases; 

 but formidable, if not fatal, difficulties 

 stand in the way of its acceptance as a 

 general principle of interpretation. In the 

 case of the bee, for example, as Castle him- 

 self pointed out, if it be assumed that the 

 female element is uniformly eliminated in 

 the maturation of the parthenogenetic egg, 

 the female element must be reintroduced 

 by the spermatozoon; but the spermatozoa 

 are produced by males that arise from 

 parthenogenetic eggs, which by the hypoth- 

 esis have eliminated the female element. 

 Castle ingeniously endeavors to meet this 

 difficulty by taking refuge in the conclu- 

 sion of Petrunkewitsch that the testes are 

 not formed from the egg proper but from 

 a fusion-nucleus formed by union of two 

 polar nuclei, in which the female element 

 is present; but until decisive evidence 

 is available that the testes really have 

 such an origin in the male bee it seems 

 to me impossible to regard the explanation 

 with anything but skepticism. But better 

 and more direct evidence than this, free 

 from any hypothetical element, is afforded 

 by the observations on aphids, recently 

 brought forward by Miss Stevens. If her 

 conclusions are well founded, as they seem 

 to be, in these animals no process of syn- 

 apsis or reduction occurs in any of the 

 parthenogenetic eggs, whether they pro- 

 duce males or females; tliovigh the sexual 

 egg's and the spermatocytes undergo reduc- 

 tion in typical fashion. The principle of 

 elimination here appears to be ruled out of 

 court as a sex-determining factor, and it 

 only seems possible to explain the result 

 by the assumption that throughout the 

 summer broods there is a uniform domi- 

 nance of the female element, and that males 

 are produced from eggs in which a reversal 

 of dominance takes place. That something 

 of the kind occurs is indicated by the fact 

 that both in aphids and in daphnids the 

 same parthenogenetic mother may produce 



both male and female offspring ; and in the 

 daphnids the condition in which this oc- 

 curs is shown by the recent experiments 

 of Issakowitsch to arise in response to a 

 change of the environment. 



Whether similar considerations of domi- 

 nance and recessiveness will afford a gen- 

 eral explanation remains quite an open 

 question, but they seem at least sufficiently 

 plausible to be taken as a convenient work- 

 ing hypothesis. By its aid we can work 

 out on paper a formal explanation of the 

 mechanism of sex production that will in- 

 clude nearly all known cases, and will also 

 include the determination of sex by ex- 

 ternal conditions (if it be admitted that 

 such a process takes place). It would not, 

 I think, be profitable to go into such specu- 

 lative constructions in detail here. They 

 are but fireside dreams which may serve a 

 useful purpose in the safe seclusion of 

 the study, but really belong in the same 

 Umbo with the so-called 'fool experiments' 

 which all of us at times secretly practise. 

 I dare say the general view that I have 

 briefly sketched will appear to some as only 

 a restatement of the problem done over 

 into the Mendelian jargon. I venture to 

 think, however, that it is a little more than 

 this. A real advance has been made if it 

 has become possible to connect sex produc- 

 tion with a definite nuclear mechanism that 

 gives us a tangible handle by which to take 

 hold of the problem. But I hardly need 

 add that this should not be considered as 

 giving more than a tentative point of at- 

 tack. It is entirely possible that we are 

 on a wrong track, that the so-called sex 

 chromosomes are only associated in a defi- 

 nite way Avith the sexual characters, and 

 have in themselves no causative influence 

 on sex production. The whole chromosome 

 theory of heredity, for that matter, stands 

 unproved before the judgment seat. I re- 

 peat, therefore, that the subject is not yet 

 ripe for discussion ; and what we need now 



