IV 



EGGS AND LARV.E AND THEIR DEVELOPMENT 87 



itself. These differences do not correspond in all cases to the 

 differences between the adult fishes, although sometimes certain 

 peculiarities of the egg are found in all the members of one 

 family and not in other families. 



The character of the enclosing membrane is important, 

 because it has a good deal to do with the conditions under which 

 the eggs are placed during development. With respect to these 

 conditions the eggs are of three kinds: (i) those which are 

 heavy, but free or separate ; (2) those which are heavy and 

 attached ; (3) those which are light and separate. The last kind 

 occur only in the sea. 



Of the first kind the salmon family supply the most im- 

 portant instances. In eggs of this kind the outer surface of the 

 egg-membrane is smooth, without projections, and hard, so that 

 the eggs have no tendency to adhere to one another or to sur- 

 rounding objects. The eggs of salmon and trout are deposited 

 in the gravel of the bottom of running streams, the female fish 

 digging a trough in the gravel, in which she deposits the eggs, 

 and then heaping more gravel over them. The eggs of the 

 salmon are about one-fourth of an inch in diameter. The eggs of 

 the shads also lie at or near the bottom in a free condition, and 

 are deposited in the tidal water of estuaries, or in the fresh water 

 of rivers. The only other case among British fishes is that of 

 the turbot (Lota vulgaris), which, though a fresh-water fish, 

 belongs to the cod family. Its eggs lie loose and separate at the 

 bottom of the water. 



Heavy adhesive eggs are laid by nearly all other fresh-water 

 fishes, and by a great many marine fishes which live near the 

 shore. In the majority of these the surface of the egg-membrane 

 is smooth but sticky when the egg is first laid, afterwards harden- 

 ing and so firmly attaching the eggs to one another or to other 

 objects in the water. The eggs of the sturgeon are attached to 

 stones, &c., at the bottom of rivers. The eggs of the carp family, 

 all of which live in fresh water, are attached to water plants, and 

 so also are those of the pike and other members of the pike 

 family. The eggs of the herring are similar in this respect. 

 Each egg of this fish is six-hundredths of an inch in diameter. 

 The spawn is deposited on rough gravelly or stony ground, or 

 shingly banks in the sea at various distances from the shore, or 

 in certain cases, as in the Baltic, in estuaries where the water 



