18 



THE NUCLEUS IN MATURATION 



the lieter atypical, the second as the homotypical, division. The post-reduction phase 

 is represented in animal forms by the mature sex-cells, but in certain plants in 

 which there is alternation of generations it includes all the cells of the sexual genera- 

 tion. The post-reduction phase comes to an end with the union of the nuclei 

 of the sex-cells, and it is obvious that as each conjugating nucleus has only one- 

 half the number of chromosomes possessed by the somatic cells, the somatic 



FIG. 27. DIAGRAM OF FERTILISATION. (After Boveri.) 

 The number of chromosomes is four : paternal, red ; maternal, blue. 



number of chromatin-rods is restored in the segmentation-nucleus 

 formed by the union of the sperm- with the germ-nucleus. 



The phenomena are seen with greater clearness in the sperm-cells than in the 

 germ-cells, as there are certain modifications of the process in the maturation of 

 the ovum which will be alluded to later. 



The prophase of the first or heterotypical division is very prolonged, and the 

 chromatin undergoes changes too intricate to be followed in detail here. One 



