28 HEREDITY AND ENVIRONMENT 



by a complicated process known as mitosis, or 

 indirect nuclear division (Figs. 4 I-Lj 5) . The 

 centrosome, which usually accompanies the 

 sperm nucleus in its passage through the 

 egg, divides and forms a spindle-shaped figure 

 with astral radiations at its two poles (Figs. 

 3-5 ) . The chromatin, or stainable substance 

 of the nucleus, takes the form of threads, the 

 chromosomes ( Fig. 5 ) , of which there is a con- 

 stant number for each species of animal and 

 plant. Each chromosome then splits length- 

 wise, its two halves moving to opposite ends 

 of the spindle, where the daughter chromo- 

 somes fuse together to form the daughter 

 nuclei. In this way the chromatin of the egg 

 and sperm nuclei is exactly halved. 



After the germ nuclei have divided in this 

 manner the entire egg divides by a process of 

 constriction into two cells (Fig. 5 F). This 

 is the beginning of a long series of cell divi- 

 sions, each of them essentially like the first, by 

 which the egg is subdivided successively into 

 a constantly increasing number of cells. Dur- 

 ing the earlier divisions there is little or no in- 

 crease in the volume of the egg, consequently 



