DEVELOPMENT OF THE FERTILIZED EGG 281 



an egg nucleus seems to endow it with renewed power. The 



nucleus of the egg previous to fertilization has lost its power of 



division, and if left to itself, eventually dies 



and disappears; but after fusing with the 



sperm the combined nucleus shows a rein- 



vigorated power of growth. It begins almost 



at once to divide in two parts (Fig. 132 A); 



the process of the division of the nucleus fol- A j 



lowed by the division of the cell is identical 



with that described on page 85. As a result 



of this division there are produced two cells, FlG 131 



each with a centrosome, each with its PRODUCTIVE 

 nucleus, which contains the same number of CELLS OF 

 chromosomes as the fertilized egg nucleus. FROG 

 Moreover, at the beginning of the division, sp ^ r ' m egg; B ' the 

 each chromosome is split lengthwise, and half 

 of each chromosome passes into each of the two nuclei 

 of the two new cells. Each of the two cells thus contains 

 chromatin material from each of the chromosomes of the 

 fertilized egg, and since these chromosomes come partly from 

 the male and partly from the female parent, it follows that 

 one-half of the chromatin in each cell is derived from the 

 male, and one-half from the female parent. Hence, each cell 

 will contain inherited traits from each parent. This first divi- 

 sion of the egg is soon followed by a second, which produces 

 four cells, and in this division the same process is repeated, 

 the chromatin material being again split up so that each of 

 the four cells (Fig. 132 A) contains chromatin material from 

 both parents. This process now goes on, the cells dividing 

 again and again, until the original egg has divided into a large 

 number of small cells, each cell probably containing chromatin 

 material from both parents. This process of segmentation or 

 cleavage is the first step by which all animals and plants begin 

 their life history, the egg in all cases dividing after a similar 

 manner into a large number of cells. 



