x] Gametes in Groups of Four 195 



are not all equally possible, and that fertilisation can only 

 take place between gametes dissimilar in respect of sex- 

 factors. As yet there is, I think, no direct evidence for 

 this view. In my judgment the fact most favourable to 

 this conception is the almost universal arrangement of 

 gametic systems in groups of four. The maturation-pro- 

 cesses of male cells, and, with very rare exceptions, those 

 of ova which are to undergo fertilisation, take place in such 

 a way that a group of four, not two, reduced nuclei results. 

 Both pollen-grains and spermatozoa are arranged in tetrads. 

 The ovum prepared for fertilisation by the maturation- 

 processes has ejected, except in a few special cases, three 

 nuclei comparable with itself (though very often two of 

 these may by an equation-division be extruded in combi- 

 nation). Such differentiation by groups of four is strongly 

 suggestive of the possibility that the four parts are not all 

 comparable. It may be that the simple allelomorphism 

 we so often find is really a phenomenon of simple cases 

 only, and that the fundamental differentiation is in reality 

 dimorphic for each sex. The gametic series for the hetero- 

 zygote may not be A, a, A, a, but A, a, A', a' both on the 

 female and male sides ; and this may be the meaning of the 

 grouping into sets of four, not sets of two. Naturally how- 

 ever very cogent evidence must be produced in order to 

 establish such a proposition as this. 



