FISH 



467 



numbers of them. The fish that escape make their way farther 

 upstream, where they finally deposit eggs. The young which 

 hatch from the eggs remain in the fresh water until they are large 

 and strong enough to make the journey back to the sea, where 

 they stay until they become adults and are able to spawn. 



Reproduction among fish is very different from that of either 

 mammals or birds. At the spawning season, which is usually 

 spring or early summer, the female discharges eggs from her 

 body, and the male 

 deposits a milky sub- 

 stance called milt 

 over them. In order 

 that eggs may hatch 

 into young fish, it is 

 necessary that they 

 come in contact 

 with sperm cells in 

 the milt. If eggs 

 are carried away 

 by waves and wind 

 before milt comes in 

 contact with them 

 and fertilizes them, 



,,.11 FIG. 337. Stripping and fertilizing eggs at a hatchery. 



they fail to hatch and 



quickly decay. The number of eggs laid by fish is enormous, 

 a single trout laying thousands, a single shad hundreds of 

 thousands, and a single cod several millions of eggs. The roe of 

 shad, which is such a popular dish in the spring, is the egg mass 

 of the female shad. If the female shad is not caught, but is 

 left in the water, the skin of the roe bursts and sets free the eggs, 

 which escape into the water and, if fertilized, develop into young 

 shad. Of all the vast number of eggs produced by the female, 

 few develop into mature fish, because many of the eggs perish 



CLARK INTRO. TO SC. 30 



