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'"*. A fact perhaps favourable to this conception is the 

 almost universal arrangement of gametic systems in groups 

 of four. The maturation-processes 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 oi 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 combination). 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 heterozygote 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 however very cogent evidence must 

 be produced in order to establish such a proposition as this. 



* There is one piece of direct evidence strongly suggestive of di- 

 morphism among spermatozoa. It was mentioned (p. 172) that the 

 daughters of colour-bhnd men can transmit the affection, while the sons 

 of these same fathers are free from it and cannot transmit it. Until 

 proper statistics are forthcoming it is not possible to build with complete 

 confidence on this fact, but it is a clear indication of dimorphism among 

 the sperms, such that those destined to take part in the production of 

 females bear the colour-blindness factor, while those destined to fertilise 

 the male ova are free from this factor. We do not yet know that all the 

 daughters of colour-blind men can transmit, but the records are I think 

 consistent with the belief that they may. While holding to the view 

 expressed in the text that the female is heterozygous in femaleness {F) we 

 may perhaps suppose that the male is heterozygous for maleness {M). We 

 thus avoid the difficulties entailed by the theory that both sexes are hetero- 

 zygous in "sex" (see p. 165), for two allelo7norphic pairs are now involved, 

 ^and its absence, J/ and its absence. The eggs may be represented as 

 ® and O; the sperms as ^ and 0> so that in fertilisation the union 

 is always between 7^ and a blank sperm or between J/ and a blank ovum. 

 The spurious allelomorphisms described in the canary. Abraxas grossu- 

 lariata and the silky fowl are cases in which 7^ repels certain factors, while 

 in colour-blindness there may be an exactly similiar spurious allelomorphism 

 between Al and the factor for colour-blindness. 



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