THE CARRIERS OF THE HERITAGE 



activity reaches old age, and death completes the 

 cycle. In most instances, however, before this final 

 phase is reached, the cell gives place to daughter- 

 cells through fission, after the manner of most proto- 

 zoans, and a new cell cycle is begun. 



Sometimes the road of differentiation has been trav- 

 eled so far that it is apparently impossible, as in the 

 case of the complicated brain-cells, to retrace these 

 steps of differentiation and begin again. In such in- 

 stances the outfit of cells provided in the embryo 

 determines the numerical limit of the cells available 

 throughout life. When this supply is exhausted no 

 more cells appear to replace those which have been 

 worn out. 



4. MITOSIS 



The ordinary process by which two cells are made 

 out of one is termed mitosis. It occurs constantly, 

 and particularly during growth, in all cellular organ- 

 isms. A series of diagrams, modified from Boveri, illus- 

 trating the typical phases of mitosis is given in Figures 

 48 to 57. 



The restmg cell (Fig. 48) is characterized by the 

 presence of a nuclear membrane, a single centrosome, 

 and by a chromatin network within the nucleus. In the 

 beginning of the pro phase (Fig. 49) the centrosome 

 has divided into two parts, while in the early prophase 

 (Fig. 50) the two centrosomes have moved farther 

 apart and definite separate chromosomes have formed 

 out of the chromatin network. The prophase^ proper 

 (Fig. 51) is marked by the vanishing of the nuclear 



