THE HEREDITY OF SEX 51 



SEX AND SECONDARY SEXUAL CHARACTERS 



The mystery of sex is hidden ultimately in the 

 phenomenon of conjugation, that union of two 

 cells which in general seems necessary to the main- 

 tenance of life, to be a process of rejuvenation. 

 We know nothing of the nature of this process, or 

 why in general it should produce a reinvigoration of 

 the cell resulting from it. We know little if anything 

 of the relation between the two conjugating cells 

 or gametes, of the real nature of the attraction 

 that causes them to approach each other and 

 ultimately unite together. We have, it is true, 

 some evidence that one cell affects the other by 

 some chemical action, as for instance in the fact 

 that the mobile male gametes of a fern are attracted 

 to a tube containing malic acid, but this may be 

 merely an influence on the direction of movement 

 of the male gamete, while there are cases in which 

 neither cell is actively mobile. What we know in 

 higher animals and plants is that each gamete 

 contains in its nucleus half the number of chromo- 

 somes found in the other cells of the parent, and 

 that in the fertilised ovum the chromosomes of both 

 gametes form the new nucleus, in which therefore 

 the original number of chromosomes is restored. 



The remarkable fact is that from this fertilised 

 ovum or zygote is developed usually an individual 

 of one sex or the other, male or female, other cases 

 being comparatively exceptional, although each 

 act of fertilisation is the union of the two sexes 

 together. Various attempts have been made to 

 prove that the sex of the organism is determined by 

 conditions affecting it during development sub- 



