THE EVOLUTION OF SEX 41 



egg does not begin to segment until some time 

 after it has been restored to its natural 

 medium. (5) Lastly, it is probable that, in 

 some cases at least, fertilisation irrevocably 

 determines the sex of the offspring into which 

 the fertilised ovum will develop. 



Looking backwards, we see that fertilisation 

 involves great waste of life-material. It must 

 therefore have some great advantage. When 

 we consider the five things above indicated 

 as involved in ordinary fertilisation, we may 

 notice that sex is sometimes determined quite 

 apart from fertilisation and previous to this, 

 that a stimulus to division may be effected 

 by some artificial change, that the ovum has 

 originally a centrosome of its own w r hich it 

 might conceivably retain, and so on. In 

 short, there can be little doubt that the chief 

 significance of fertilisation is to be found in 

 the first two implications. There is a mingling 

 of two inheritances, which may favour varia- 

 tions and also counteract them ; and there is 

 a restoration of a certain norm of nuclear 

 constitution subsequent to a remarkable pro- 

 cess of reduction. In this reduction there is 

 a notable opportunity for variation, since the 

 number of chromosomes is reduced to half 

 the normal. Suppose the chromosomes in the 

 nucleus of an unripe egg were represented by 

 a pack of cards, the process of maturation 

 (usually the giving off of the first polar body) 

 involves a literal loss of half of the pack. The 

 normal number is restored by the addition 

 of another half-pack borne by the fertilising 

 spermatozoon. 



Green-flies or Aphides produce prolifically 



