FISHES 



139 



sperm-nucleus is forced into the ovule and unites with the 

 egg-nucleus ; this is the process known as fertilization 

 (P. B., 91). In the case of fishes the sperm-cells swim to the 

 eggs, and then force their way into the egg (Fig. 102, A). 



FIG. 104. Nest of stickleback. Above, male entering nest with eggs ; 

 below, male depositing sperm-cells. 



The nucleus of the sperm- and egg-cells then unite just as in 

 plants (Fig. 102, B, C, D). The egg nucleus thus fertilized 

 first divides, and then the cell body, and thus are formed 

 two cells. Each of these cells in turn divides, and so 

 four cells are produced (Fig. 103, A, B). The process 'of 



