24 HEREDITY AND ENVIRONMENT 



3. Cleavage. — When the two germ nuclei, 

 egg nucleus and sperm nucleus, have come 

 into contact after the fertilization of the egg 

 they divide by a complicated process linown as 

 mitosis, or indirect nuclear division (Fig. 9). 

 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. 

 7, 8). The chromatin, or stainable substance 

 of the nucleus, takes the form of threads, the 

 chromosomes ( Fig. 9 ) , 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, in which position the daughter 

 chromosomes fuse together to form the daugh- 

 ter 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 (Figs. 10, 11) . 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 



