CHAPTER XIV 



The Mechanism of Hereditary Transmission 



ESPECIALLY since WEISMANN brought forward the first 

 \ outline of his theory of heredity in 1885, one of the 



chief motives and objects of cytological investigation has 

 been the discovery of the means by which inherited charac- 

 ters are transmitted from parent to offspring. We know that 

 in general the father and mother contribute equally to the 

 hereditary features of their children, and yet in Man the 

 whole of this almost infinitely complex web of family like- 

 ness is transmitted, on the father's side, by a spermatozoon 

 about one five-hundredth of an inch in length 1 . If the facts 

 of heredity were not so familiar, they would be regarded as 

 the most wonderful thing in nature, and it is not surprising 

 that one of the chief aims of cytologists has been to find some 

 explanation of their mechanism. 



WEISMANN'S famous theory of the germ plasm was largely 

 instrumental in leading to more thorough investigations of the 

 behaviour of the chromosomes, for according to his scheme 

 these bodies bore the "ids" or determinants which were the 

 basis of hereditary transmission. It is not intended here to 

 discuss these older hypotheses; much of WEISMANN'S com- 

 plicated theory is already disproved and almost forgotten, 

 but it is impossible to consider more recent ideas on the 

 subject without referring to the fact that he not only pro- 

 vided the stimulus which has led to such brilliant results 

 in the last few years, but also first gave sound reasons for 



1 A human spermatozoon is approximately '05 mm., or about 5^5 inch, in 

 length, and of this length fully nine-tenths is tail. 



