MULTIPLICATION IN ANIMALS 



485 



A sperm cell of a fish is shown below in 4, Fig. 203. When the 

 nucleus of the egg has fused with the nucleus of the sperm, 

 the combined nucleus begins to divide, and thus the develop- 

 ment of a new fish is started. 



The female gamete of the fish contains a small amount of 

 food material in addition to the protoplasm. Upon this food the 

 hatching fish lives until it has developed far enough to be able 



Fig. 203. Sperm cells of animals. (Very highly magnified) 

 I, pig; 2, bird; 3, salamander; 4, ray; 5, threadworm {Ascaris); 6, lobster 



to get its own food. In some species the adults swim about in 

 the neighborhood of the developing fry and protect them from 

 possible destruction by other fish. A few species, like the 

 stickleback (Fig. 204), prepare a rather rough nest for the eggs. 

 With most fishes, however, the sperms and eggs are thrown into 

 the water by the adults and then left to themselves. Thus ex- 

 posed, thousands of eggs are destroyed before they have a chance 

 to develop into fish. Of course, thousands are also destroyed in 

 the case of those species that protect their young, but it is likely 

 that the proportion of loss is not so great among the latter. 



