356 METAZOAN PHYLA 



If laid in masses, they are less likely to be eaten. The young fish, or 

 fry, are constantly exposed to destruction. Though enormous numbers 

 of eggs are produced, amounting to many millions for each individual 

 during the breeding season, only a relatively small number produce 

 young that reach maturity. Some fish are viviparous, both fertilization 

 and development being internal and the young being born with the 

 characteristics of the adult. The young of oviparous forms often differ 

 in appearance from the parents, and sometimes the change from larval to 

 adult characters is so pronounced as to amount to a metamorphosis. 



The egg is telolecithal, and discoidal cleavage takes place (Fig. 242). 

 The division of the protoplasmic area at the top, called the germinal 

 disc, results in the formation of a germinal area, or blastoderm, which 

 at first forms a disc on the upper side of the egg. As development 

 proceeds, the blastoderm spreads out, gradually grows around the egg, 

 and comes to inclose the yolk completely. While the germinal disc is 

 still confined to the upper side of the egg, a thickening appears at the 

 margin of this disc, which is more marked at the point that will form 

 the head of the embryo. From this point a raised strip of tissue runs 

 backward in the median line. This strip is marked lengthwise by 

 two lateral ridges bounding a median groove which is the medullary, 

 or neural, groove. The embryo, which thus develops on the upper side 

 of the egg, rises higher and higher above the surface, the yolk being 

 at the same time gradually used up as food. The neural groove becomes 

 a neural tube by the meeting of the lateral margins and so the central 

 nervous system arises. The head becomes free at the anterior end of 

 the embryo and a tail forms at the opposite end. As the length increases, 

 various structures characterizing the fully developed fish make their 

 appearance, and the young animal hatches with the underside of the 

 body distended by the yolk still remaining. The yolk is soon used 

 up, but by that time the fish is sufficiently developed to secure 

 its own food, and it begins to eat the more minute forms of life in the 

 water, increasing the size and variety of its food as it grows larger. 

 It is evident from this description that the entire egg ultimately con- 

 tributes to the body of the fish. 



During the breeding season temporary mating in some cases occurs 

 and the pair may cooperate in the construction of a nest and the care 

 of the eggs and young. The males of some marine forms have pouches 

 for the carrying of eggs during development. This indicates the wide 

 diversity in behavior during reproductive activity. 



386. Ages of Fish. — Some fish, such as the ice fish of China, live only 

 a year. Such short-lived fish have a definite size which they may attain. 

 Others, like salmon, live only a few years and die at the first reproductive 

 period, after spawning. Still others live for a long time, it is believed 

 for from twenty to thirty, and perhaps even sixty, years and continue 



