No. 4So] 



THE 1)I-:TI:NM IXATIOX OF SEX 



311 



tables whether duplicate twins wvw fxcliidcd, as thcv slioiild have 



been excluded, the probability iliat ..nliiiarv twins woiiM l)e of 

 the same sex nii<^ht have bccMi even less than .77; for diij)licate 



Sex determined before the first cleavage of the ovum. ].v\ us 

 now examine into the bearing of these con.sideratioiis upon New- 

 comb's hypothesis. To continue his simile, two j)arti( lcs stariiiiij; 

 together will have a greater chance of remaining touctlier and 

 passing on the same side of the barrier than two particles somewhat 

 removed from each other. Himilarly. two ova (lcveloj)ing together 



offspring of the same sex than two ova developing at ditrerent 

 times, but neither the two particles nor the two ova iiivariablv 

 follow the same course. If any series of accidents acting upon the 

 ovum after fertilization is to determine the sex of the twins, it is 

 incredible that it should always ])r()duce the same re>uh in lioth. 

 Since, however, duplicate twins ai-e always of the same <e\, this 

 view becomes untenable, and we must limit the action of a series 

 of accidents to the ])eriod j)receding and j)ossibly including fertili- 

 zation. The conclu.sion that at or inunediately after fertilization, 

 the sex of the offspring is determined once for all seems inevitable. 

 The effect upon the ovum of any series of accidents must cease 

 before the first cleavage is accomplished. 



Is sex determined by either parent alone?— Having concluded 

 that the sex of the offspring is determined at or before the time 

 of fertihzation, we may inquire further whether the sex of the 

 offspring may not be determined by the ovtiin alone, or by the 

 spermatozoon alone. First, the ovum may have the potentiality 

 of developing into either a male or a female embryo. Dnring 

 maturation the chromatic material neces.sary for the (iev(>loj)inent 

 of an embryo of one sex is cast off in the polar bodiis. and that 

 necessary for an embryo of the other sex is retained. The sperma- 

 tozoon thus plays a purely asexual role. This hypothesis ])o>tH- 

 lates a qualitative reduction of the chromatin in maturation. 

 According to another variety of this hypothesis the ovmn assumes 

 the asexual role, and the sex of the embryo i> .letermined solely 

 by the spermatozoon. Since all spermatozoa do not [)roduce 



