THE CELL AND PROTOPLASM. 



103 



is the egg. But before its division can begin 

 there occurs in all cases of sexual reproduction a 

 process called fertilization, the essential feature 

 of which is the union of this cell with another 

 commonly from a different individual. While the 

 phenomenon is subject to considerable difference 

 in details, it is essentially as follows : 



The female reproductive cell is called the egg, 

 and it is this cell which divides to form the next 

 generation. Such a cell is shown in Fig. 33. 

 Like other cells it 

 has a cell wall, a 

 cell substance with 

 its linin and fluid 

 portions, a nucleus 

 surrounded by a 

 membrane and con- 

 tain ing a reticulum, 

 a nucleolus and 

 chromatic mate- 

 rial, and lastly, a 

 centrosome. Now 

 such an egg is a 

 complete cell, but 



it is not able to FIG. 33. An egg showing the cell sub- 

 be "in the process stance, and the nucleus, the latter 

 r & j. . . containing chromosomes in large 

 Of division Which number and a nucleolus. 



shall give rise to a 



new individual until it has united with another cell 

 of quite a different sort and commonly derived from 

 a different individual called the male. Why the 

 egg cell is unable to develop without such union 

 with male cell does not concern us here, but its 

 purpose will be evident as the description pro- 

 ceeds. The egg cell as it comes from the ovary 

 of the female individual is, however, not yet ready 

 8 



