140 HEREDITY [ap. 



therefore impossible according to his theory, that 

 'acquired characters' in the technical sense should 

 ever be inherited. The germ-plasm of one genera- 

 tion gives origin to the germ-plasm of the next, and 

 no external conditions acting on the body which 

 contains and nourishes the germ-plasm can have 

 effects which are transmitted unless the germ-plasm 

 itself is altered. 



Weismann in a series of books and papers has 

 built up a detailed and highly complicated and 

 speculative scheme of the nature and composition 

 of the germ-plasm, only a brief summary of which 

 can be given here. Much of it will doubtless not 

 stand the test of fuller investigation, and parts of 

 it are already discredited ; but it has had the merit 

 of stimulating an immense amount of valuable re- 

 search, and there are indications that some of his 

 fundamental ideas will form the foundations of a 

 true theory of the material basis of heredity. 



Weismann assumes that the germ-plasm is con- 

 tained in the nucleus of the cell, and, in particular, 

 in the bodies known as chromosomes. Every nucleus 

 contains a number of these bodies, in the ordinary 

 condition of the nucleus distributed through its 

 substance so as to be unrecognisable, but when the 

 cell is about to divide they make their appearance 

 as rod-like bodies whose number in general is con- 

 stant in the nuclei of the same species of animal or 



