18 GENETICS 



period of normal activity old age ensues, 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 protozoans, and a new cell cycle is begun. 



Sometimes the road of differentiation has been 

 traveled 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 instances the outfit of cells provided in the em- 

 bryo determines the numerical limit of the cells 

 available throughout life. When this supply is ex- 

 hausted no more cells appear to replace those w^hich 

 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, 

 illustrating the typical phases of mitosis is given 

 in Figures 4 to 13. 



The resting cell (Fig. 4) 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 prophase (Fig. 5) the centrosome 

 has divided into two parts, while in the early prophase 

 (Fig. 6) the two centrosomes have moved farther 

 apart and definite separate chromosomes have formed 

 out of the chromatin network. The prophase proper 

 (Fig. 7) is marked by the vanishing of the nuclear 



