PROBLEM 2. How More Complex Aniiimls Reprodiice 



427 



Fig. 374 7/.V inale sunfish guards the nest he 

 has dug in the sand, (encyclopaedia britannica 



FILMS, INC.) 



As the fish develops, sex organs are 

 formed partly from these unchanged 

 cells. Then, when the fish has grown to 

 full size, the unchanged cells in these sex 

 organs develop into eggs or sperms. In a 

 female there will be eggs; in a male, sper- 

 maries have grown and the gametes are 

 sperms. Thus you see that, while the 

 sperms and eggs are found only in the 

 fully developed fish, the cells from 

 which the gametes came were already 

 present in the very young embryo. 



What protection do parent fish give 

 their young? Many kinds of fish deposit 

 eggs and sperms and leave them unpro- 

 tected and exposed to the elements and 

 to many enemies. A few kinds scoop out 

 a small depression in the sand or gravel 

 under the water and deposit eggs and 

 sperms there. Probably many eggs fail to 

 develop because no sperms reach them. 



Fig. 375 The nest of the stickleback is jnade 

 of water weeds glued together with a sticky 

 substaTice secreted by glands in the body of 

 the male. He guards the eggs that the female 

 places in the nest, (new york aquarium) 



Of those that are fertilized countless 

 numbers are destroyed, for the parent 

 rarely stays to protect the young. Some 

 of the exceptions are interesting. In one 

 kind of fish, embryos are carried about 

 in the mouth of the male parent. Even 

 after they are sizable fish they keep re- 

 turning to his mouth when danger threat- 

 ens. How often he yields to the tempta- 

 tion of hunger no one knows. Another 

 exception is the male stickleback which 

 builds a true nest and guards the eggs. 



Frog reproduction. In the early spring 

 frogs and toads lay their eggs in quiet 

 waters. When mating occurs, the male 

 frog is above the female so that as the 



