282 



A HISTORY OF FISHES 



be very closely packed together at this season. Analysis of 

 these shoals generally shows the females to be in greater numbers 

 than the males, but this is by no means always the case. Here 

 the individuals of both sexes simply discharge their ova and 

 sperms into the water and the fertilised eggs are subsequently 

 abandoned by the parents and left to the mercy of physical 

 conditions. The actual rate at which the eggs are extruded 

 varies a good deal in the different species, in some cases all or a 

 large proportion of the ova being ripe for fertilisation at more 



or less the same time, 

 while in others the 

 process is compara- 

 tively slow and only 

 a certain number 

 ripen and are ex- 

 truded at one time. In 

 the Cod {Gadus) and 

 Plaice {Pleuronectes) . 

 indeed, in all our 

 food-fishes, with the 

 exception of the Her- 

 ring {Clupea) and 

 Shad {Alosa)^th& eggs 

 are minute and buoy- 

 ant, and float at or 

 near to the surface of 

 the sea. Under such 

 conditions the hap- 

 hazard mode of re- 

 production must 

 inevitably lead to a 

 great waste of sex- 

 cells, but this is dim- 

 inished to a certain extent by the fact that the fishes are closely 

 congregated and that both eggs and sperm will float roughly at the 

 same rate and in the same direction. Drifting about in the sea at 

 the mercy of the wind and currents, many of the eggs serve as food 

 for other fishes, many are killed by changes of temperature 

 and other physical catastrophes, and many more are cast on 

 the shore by adverse currents. Mr. Masterman, writing of the 

 Cod's eggs, remarks: " It is evident that for the successful 

 development of the young fish a concatenation of favourable 

 circumstances is necessary, which depends in the main upon 



Fig. 104. 

 Heads of male breeding Salmon (Salmo). 



A. Atlantic Salmon (Sahno salar), X |: ; b. Pacific. 

 Blue-back Salmon (Salmo nerka), X ^. 



