140 



ANIMAL BIOLOGY 



division continues until a many-celled organism is de- 

 veloped. 



As the cells increase in number, they become different in 

 character and form the various organs of the body. When 

 the little fish first hatches, and begins to swim about, it 

 often has attached to it some of the food substance (yolk) 

 stored in the egg (Fig. 103, D). After this is used up, the 

 young fish must secilre its own food. 



Most fishes do not take any care of their eggs or young, and 

 in some cases the parents die soon after the eggs are laid and 

 fertilized. In the case of the stickleback, however, the male 

 fish makes a nest (Fig. 104) in which the females deposit their 

 eggs. The male then extrudes sperm over the eggs. The 

 male stays about .the nest and guards the eggs and also 



the young sticklebacks 

 when they hatch out. 



105. Artificial prop- 

 agation of fishes. — 



Since, as we have said, 

 most kinds of fishes 

 give no attention to 

 eggs or young, enor- 

 mous numbers of both 

 eggs and young are 

 eaten by other fishes; 

 hence, only a small pro- 

 portion come to ma- 

 turity. For example, 

 while a codfish lays 8,000,000 eggs, only about two of 

 these eggs on the average come to maturity. Hence, in 

 order to increase to any considerable extent the number of 

 fishes, the eggs are artificially hatched. That is, the fish 



Fig. 105.- 



-Artifioijl fertilization of eggs. 

 (Coleman.) 



