I] THEORIES OF HEREDITY 141 



plant. Before the nucleus divides the chromosomes 

 split longitudinally so that they are accurately halved 

 and the two halves of each go into different daughter- 

 nuclei. Weismann supposes that the germ-plasm 

 is contained in the chromosomes, and consists of 

 numerous units with different properties. When the 

 chromosome splits, each unit is supposed to divide 

 into two similar halves, and thus each daughter- 

 nucleus receives a similar complement of germ-plasm. 

 In the union of male and female cells in fertilisa- 

 tion, the nucleus of each cell brings its complement 

 of chromosomes, and thus if there were no special 

 provision, the number of chromosomes would be 

 doubled in each generation. But it is actually found 

 that in the cell-divisions immediately preceding the 

 development of both male and female sex-cells, a 

 process occurs which results in the removal of half 

 the chromosomes from the nucleus, and thus when 

 the male and female nuclei unite the normal number 

 of chromosomes for the species is restored. Since 

 Weismann regards the chromosomes as consisting 

 of germ-plasm, and as made up of a vast number 

 of units, each of which is the determinant for one 

 hereditary character, he saw that, without some such 

 process of removal of chromosomes in the formation 

 of the sex-cells, the germ-plasm must in a few genera- 

 tions become infinitely complicated. He therefore 

 predicted that some process of 'reduction' of chromo- 



