FACTORS IN SEX-DETERMINATION 485 



into females only ; in other cases, e.g. tadpoles, it seems as if 

 the young offspring pass through a sexually neutral or potentially 

 hermaphrodite stage, during which environmental conditions 

 may give them a bias towards maleness or femaleness. In some 

 cases we know that an unfertilised egg will certainly give rise to 

 a male, as in the eggs which develop into drone bees ; in other 

 cases we know that an unfertilised ^gg will as certainly give rise 

 to a female, as in the aphides throughout the summer months. 

 Although recent work points more and more clearly to the 

 conclusion that in a large number of cases the sex is absolutely 

 fixed in the fertilised ovum or even earlier, yet the variety of 

 organisms is so great that it seems at present rash to exclude 

 the possibility that nurtural conditions before and after birth 

 have an influence on the sex of the developing organism. 



Among the considerations which suggest that the future sex 

 of the organism is often determined in the fertilised o.g'g, we may 

 mention the alleged fact that the sex of " identical " twins is 

 always the same. The interest of this is that identical twins 

 appear to be produced from one ovum which becomes separated — ■ 

 at an early stage — into two independently developing halves. 

 The material being identical to start with, the results are identical. 

 In the case of ordinary twins, which arise from two distinct ova, 

 the sex may be the same in the two, or one may be male and 

 the other female. It must be admitted, however, that this sharp 

 distinction 'between identical twins and ordinary or fraternal 

 twins is not unanimously accepted. (See Thorndike, cited by 

 T. H. Morgan [1907] p. 412.) 



In one of the armadillos {Praopus or Tatusia hyhrida), Jhering 

 found in two cases eight embryos in a common chorion which 

 were all males. Arguing from the inclosure of identical human 

 twins in a common chorion, Jhering concluded that these eight 

 embryos were aU derived from one ovum, and that this was the 

 explanation of their sameness of sex. 



In some remarkable cases of " poly-embryony " recorded by 



