Doncaster says "The general conclusion with regard to man must therefore be that if sex is determined solely by the spermatozoon there is no hope either of influencing or predicting it in special cases. On the other hand, there is considerable evidence that the ovum has some share in the effect, and if this is so, before any practical results are reached it will be necessary to discover which of two conceivable causes of sex-determination is the true one. It is possible that there are two kinds of ova, as well as two kinds of spermatozoa, and that there is a selective fertilization of such a kind that one kind of spermatozoon only fertilizes one kind of ovum, the second kind of spermatozoon the second kind of ovum. If this should prove to be the case, it is possible that means might be found of influencing or predicting that kind of ovum which is discharged under any set of conditions. Secondly, it is possible that the ova are potentially all alike, but that their physiological condition may under some circumstances be so altered that the sex is determined independently of the spermatozoon. * * * It is hardly possible to avoid the conclusion that the sex of the offspring may be influenced, at least under certain circumstances, by the mother. The search for means of influencing the sex of the offspring through the mother is not of necessity doomed to failure. No results of a really positive kind have been obtained hitherto, and some of the facts point so clearly to sex-determination by the male germ-cell alone in man and other animals that many investigators have concluded that the quest is hopeless; but until an adequate explanation has been given of certain phenomena discovered in the investigation of the subject, it seems more reasonable to maintain an open mind, and to regard the control of sex in man as an achievement not entirely impossible of realization."