No. 485] 



THE DETERMINATION OF SEX 



317 



often be of different sexes while duplicate twins are invariably of 

 the same sex if nutritive or any other conditions outside of the 

 ovum itself are responsible for the sex of the offspring. It is per- 

 haps conceivable that, in the case of ordinary twins, the placental 

 circulation may be more highly developed, and the nutrition con- 

 sequently better, for one twin than for the other, or that some 

 peculiar local characteristic of the uterine wiill inay aflVct one 

 twin more than the other. The probability of any mk Ii dissimi- 

 larity of conditions in the case of dupHcate twins, \v!ui(> tli(^ same 

 placenta supplies both w^ith nutriment, and any local peculiarity 

 of the uterine wall affects them equally, is very small. It must 

 therefore be admitted that ordinary twins may be subjected to 

 more diverse conditions during development than duplicate twins, 

 and it is conceivable that the latter might sometimes be of opposite 

 sexes if we could vary the conditions during development. This 

 would manifestly be a matter of great difficulty in mammals, but 

 a simpler method of attack is open. 



Roux ('85) and others have shown that the indivi.lual blasto- 

 meres of a frog's egg will, when separated from tlic otluM s. develop 

 into complete embryos. Such embryos are presmnahly compara- 

 ble in all respects to duphcate twins, and if by any means we might 

 cause two blastomeres from the same ovum of any animal nor- 

 mally reproducing sexually to develop into embryos of opposite 

 sexes, we w^ould have a demonstration that sex was not determined 

 at the time of fertilization of the egg. Failure to produce from 

 the same egg two embrj^os of opposite sex would be evidence that 

 w^e have, at present, no known means of changing the sex of the 

 embryo after fertilization of the egg. It is incumbent upon those 

 who maintain that sex is determined by the environment to show 

 that two embryos of opposite sexes can be produced from the 

 same ovum. The experimental solution of the ])rol.lem of the 

 causes which influence the sex of the offsprinu". as well as the 

 significance of sex itself, is to be souirhr in the >im])h> ceU whose 



It is obvious, also, that the problem of sex determination is t)ut a 

 particular phase of the much wider problem of the extent to which 

 the ovum may be modified by a change in the external environ- 

 ment. Furthermore, if we acquire experimental data on the deter- 



