in GENERATION OF FISHES AND THEIR FECUNDITY 83 



of the Plymouth Aquarium, although numbers had lived in health 

 there for years. It was found that the female fish became 

 swollen in the spawning season as though the roes were enlarged, 

 but no soles' eggs appeared in the tank, although in the same 

 tank plaice spawned freely. After a time the swelling of the 

 females began to diminish, and on killing some and opening 

 them it was found that the cavities of the roes contained the 

 dead and shrunken remains of ripe eggs. Evidently in con- 

 sequence of the conditions of confinement the eggs after devel- 

 oping were not shed in the natural way, but died within the roe, 

 and were then gradually expelled. Turbot and brill have also 

 hitherto failed to spawn in confinement. In squeezing fish to 

 obtain fertilised eggs it is important that only ripe eggs should 

 be pressed out. If too much pressure is applied some of the un- 

 ripe also escape, and can easily be distinguished by their smaller 

 size and chalk-white or yellowish appearance. 



The spawning season of a fish is the period during which 

 some individuals are found to be ripe, and it may last three, four, 

 or five months. The majority of our common sea- fishes spawn 

 in the first half of the year. The plaice is one of the earliest, 

 beginning to spawn in January and going on to the middle or 

 end of April. The cod spawns in the North Sea from February 

 to May, the sole from April to July. The herring appears to 

 have two spawning periods — one in winter or spring, one in 

 summer or autumn, in the same neighbourhood, the dates 

 differing in different districts. But there is good evidence that 

 like other fishes the same herrings spawn only once a year, and 

 that there are races which spawn in winter and others which 

 spawn in summer, the spawning grounds of the former being 

 nearer shore and nearer the mouths of rivers than those of the 

 latter. 



There is one family of fishes which forms an exception to the 

 general rule in the circumstances of its generation. This is the 

 family of the eels. Much still remains obscure and unknown in 

 the life-history of the eel and conger, but what is known is 

 sufficiently remarkable. Ripe eels have never yet been obtained, 

 all that is known of them is that they go down the rivers in 

 autumn to the sea, and probably spawn there, and that young 

 eels two to four inches long return in the following spring up the 

 rivers. It is fairly certain that the old eels die after spawning. 



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