150 THE WEALTH OF THE SEA 



a projection at each corner and are often three or 

 four inches long and two inches in width. The gaff- 

 topsail catfish has large spherical eggs about an inch 

 in diameter. The foregoing are examples of the 

 largest eggs produced in fishes. Most fishes have 

 much smaller eggs, the majority producing eggs less 

 than a tenth of an inch in diameter. 



The time required to hatch fish eggs varies from 

 several hours to a month or more. In general, small 

 eggs have a shorter period of incubation than larger 

 ones. The development of the egg is generally 

 affected by heat, as high temperatures hasten and 

 low temperatures retard development. A single de- 

 gree in temperature lengthens or shortens the incu- 

 bation period in salmon about five days. 



Newly hatched fish usually are quite transparent; 

 they have relatively large heads and eyes, and the 

 fins are not separate at first but appear as fleshy 

 folds. Generally when the young fish is a few weeks 

 to a month or so of age, it resembles the adult suffi- 

 ciently to admit of its identification. In a few 

 species, however, the young are very different from 

 the adults and pass through a metamorphosis be- 

 fore acquiring the adult form. In the eels, the lady- 

 fish, and the big-eyed herring the young are ribbon- 

 shaped and transparent. In this stage they grow to 

 a length of three inches or more. Then, in the process 

 of becoming rounded and shaped like the adult, an 

 actual shortening of the body takes place. 



The early growth period in fishes, also, frequently 

 is accompanied by changes in the mouth, the teeth, 

 the gill-rakers, and the alimentary canal. Such 



