Refroduction and Growth 169 



appears. Most conspicuous is a semi-transparent, often bright 

 red or yellow, globular mass extending along the under 

 side of the body for half of its length, which so impairs navi- 

 gation that the little animal is helpless to move, or at best 

 can shoot forward a short distance by violent wiggling of its 

 tail. This is the yolk-sac, and contains what remains of the 

 tgg material out of which the fish developed. The fry con- 

 tinues to live on this for some time 5 in some species the 

 mouth has no opening at the time of hatching, and begins to 

 function only after this rich embonpoint has been absorbed. 

 Not until then does the fry have to start scratching for its 

 own living. In fact, it is not until then that it is capable of 

 moving enough to secure its own food. 



In a few species there is a great difference in appearance 

 between the young and the mature fish. An outstanding ex- 

 ample is the common eel. Its young was for years thought 

 to be a fish of an entirely different species, and the physio- 

 logical processes which accompany its transformation to the 

 adult condition are almost as complicated as in insect larvae. 



But in the majority of species the young are enough like 

 the adult to be easily recognizable. Most of them differ in 

 color or in color pattern from their parents, and also in the 

 proportions of the skeleton, but aside from the gradual 

 changes in these features, about all that happens from the 

 time a fish graduates from the yolk-sac until it reaches ma- 

 turity is that it grows. Fish, unlike birds and mammals, 

 never entirely stop growing. The habit of establishing a 

 definite size limit, beyond which the animal does not go, and 

 which comes to be looked upon as the normal size for that 

 animal, seems to be associated mainly with warm-blooded- 

 ness, for in most of the cold-blooded groups such limits do 

 not exist. You may speak of a full-grown man, or a full- 

 grown horse, or a full-grown canary, but there is no such 

 thing as a full-grown oyster or a full-grown rattlesnake or a 



