SCIENCE AND THE SEA FISHERIES 233 



And so the cycle runs on. That man can break the continuity of 

 this cycle by removing all the fish from the sea is obviously impos- 

 sible, but he may interfere with it to such an extent as to make the 

 results unpleasant to himself, and many examples are to be found 

 of the evil effects which have followed in the case of land animals 

 and plants from ill-considered and excessive destruction of life. 



Another important branch of planktonic investigation is the study 

 of the distribution of the eggs of marine fish, which in the spring 

 months of the year form an important constituent of the plankton. 

 Towards the end of the year most of our adult and sexually mature 

 food-fish move out from the coasts into relatively deep water for 

 spawning purposes. The details of these migrations and the depths 

 at which spawning takes place are different for the various species. 

 At one time it was thought that spawning took place on shallow 

 banks or even close inshore, but this is now known to be incorrect, 

 except in the case of the herring, which has demersal eggs. Except 

 in a very few cases, such as the sharks and dog-fish, there is no 

 sexual connection between the adult fish at the spawning season. 

 The genital products, both ova and spermatozoa are shed into the 

 surrounding water, where fertilisation takes place. Before develop- 

 ment can proceed it is necessary that each egg should be fertilised 

 by a spermatozoan, and probably the ovum exerts some attractive 

 influence over the sperm. The fertilised egg undergoes its develop- 

 ment while floating in the upper layers of sea water, and during 

 this process is entirely at the mercy of wind and tide. The pelagic 

 egg is minute (varying from 1/32 in. in diameter in the case of the 

 solenette to 3 /16 in. in the halibut). All pelagic fish eggs have some 

 features in common. There is a non-calcareous shell and the 

 contents are divisible into two parts ; the living substance or 

 germinal disc, and the yolk upon which the developing embryo 

 feeds. The rate of development varies with the temperature, at 

 a high temperature development is more rapid. In the Baltic it 

 has been shown that the development of the egg of the flounder 

 proceeds when the temperature of the surrounding water is below 

 the freezing point (0°C), the actual temperature at which it com- 

 mences being minus i-6 C. The author has taken these eggs in 

 the Baltic when the steamer had to be put astern to break the ice, 

 so that the tow net could be lowered. At the commencement of 

 the season the larvae of the flounder are hatched in eleven days, 

 but later in the spring when the weather is warmer six days are 

 sufficient. When hatched the young fish carries a relatively 

 enormous bag — ^the yolk sac — on its ventral surface. This bag 

 contains the remains of the yolk on which the young fish continues 

 to feed for several days. When the yolk is absorbed the larva 



