THE CELL AND PROTOPLASM. 121 



machinery for controlling its division and thus 

 regulating cell division. 



FERTILIZATION OF THE EGG. 



This description of cell division will certainly 

 give some idea of the complexity of cell life, but 

 a more marvellous series of changes still takes 

 place during the time when the egg is preparing 

 for development. Inasmuch as this process still 

 further illustrates the nature of the cell, and has 

 further a most intimate bearing upon the funda- 

 mental problem of heredity, it will be necessary 

 for us to consider it here briefly. 



The sexual reproduction of the many-celled 

 animals is always essentially alike. A single one 

 of the body cells is set apart to start the next 

 generation, and this cell, after separating from the 

 body of the animal or plant which produced it, 

 begins to divide, as already shown in Fig. 8, 

 and the many cells which arise from it eventually 

 form the new individual. This reproductive cell 

 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 differ- 

 ence 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 sub- 

 stance with its linin and fluid portions, a nucleus 



