64 MENDELISM AND 



following reduction division, give rise to ova, while 

 an ovum containing one X sex-chromosome, or 

 two different, XY, chromosomes, at the next re- 

 duction division gives rise to spermatozoa. The X 

 sex-chromosome is not in itself either female or 

 male, since, as we have seen, either ovum or sper- 

 matozoon may contain a single X chromosome. The 

 ovum then with one X clu'omosome or one X and 

 one Y changes its sex at the next reduction division 

 and becomes male. In parthenogenetic ova this 

 happens without conjugation with a spermatozoon 

 at all : in other cases, since the zygote is com- 

 pounded of spermatozoon and ovum, we can only say 

 that in the XX zygote, the ovum developing only 

 ova, the female is dominant, in the X ov XY zygote 

 developing only spermatozoa the male is dominant. 

 Hermaphrodite animals, as has been pointed out 

 by Correns and Wilson, cannot be brought under 

 this scheme at all. In the earthworms, for instance, 

 we have, in every individual developed from a 

 zygote, ova and spermatozoa developing in different 

 gonads in different parts of the body. The dif- 

 ferentiation here, therefore, must occur in some 

 cell - division preceding the reduction divisions. 

 Every zygote must have the same composition, 

 and yet give rise to two sexes in the same individual. 

 Further light on the sex problem, as in many other 

 problems in biology, can only be obtained by more 

 knowledge of the physical and chemical processes 

 which take place in the chromosomes and in the 

 relations of these structures to the rest of the cell. 

 The recent advances in cytology, remarkable as they 

 are, consist almost entirely of observations of 

 microscopic structure. They may be said to reveal 



