CHAPTER VII 



HEREDITY AND SEX 



It is now generally regarded as certain that the chromatin 

 material of the nucleus, both in the male and in the female 

 generative cells, mediates in the transmission of most, if not 

 all, of the hereditary characters. For although " cytoplasmic 

 inheritance " by means of certain self -perpetuating bodies in 

 the cytoplasm, known as plastids, is supposed by some to take 

 place in special instances, it is not claimed to be a phenomenon 

 of general occurrence, and the evidence relating to it is vague 

 and uncertain. 



That the chromosomes, or chromatin filaments of w^hich the 

 nucleus is composed, are of fundamental importance, is shown 

 by a group of wxll-ascertained facts. With certain exceptions 

 their number is constant for all the body cells in the individuals 

 of the same sex in any particular species, and when it varies 

 it does so in multiples of a definite number, though this condition, 

 which is then called " polyploidy," is uncommon. In the matura- 

 tion of the germ cells, both ova and spermatozoa, in all species, 

 the number of chromosomes is reduced to one-half (or approxi- 

 mately one-half, since one of the special sex chromosomes 

 may sometimes disappear) of the original number (see above, 

 P-36). 



The fertilised ovum, as already described, contains the full 

 complement of chromosomes. Tlje conclusion that the characters 

 of heredity are locaHsed in the chromosomes was accepted by 

 Weismann, who made it the basis of his famous theory of heredity. 

 This theory assumed (what is now widely believed) that the con- 

 jugation of the gametes is the source of variation, and that 

 acquired (as contrasted with innate or congenital) characters 

 cannot be inherited. 



The theory of Mendel, first formulated in the middle of the 

 nineteenth century, and rediscovered about twenty-five years 



