88 ^rARKETABLE BRITISH MARINE FISHES 



is SO fresh that the spawn is actually found attached to fresh- 

 water plants. Fig. 39 shows the appearance under the micro- 

 scope of a single herring egg in the living condition with parts 

 of the membranes of its neighbours to which it adheres. It is 

 worth noting that round adhesive eggs can only touch one 

 another and stick to one another at separate places, not all over 

 the surface. There are thus in a clump of eggs spaces or channels 

 left where the eggs are not touching, and where, therefore, water 

 can pass through the mass. However, in spite of this, a clump 

 or mass of adhesive eggs sticking together is more difficult to 



Fig. 39. — Living Egg of the Herring, magnified. 



keep alive than an equal number of separate eggs, in conse- 

 quence of the liability of the eggs in the clump to become 

 suffocated. Living eggs, like living fishes, require a constant 

 renewal of clean water, water containing dissolved air, and the 

 more eggs there are in a small space the more rapid the change of 

 water which is necessary to them. In natural conditions, herring 

 spawn receives a sufficient change of water from the flow of 

 the tides. The herring is the only one of the British members 

 of its family whose eggs are of the adhesive or fixed kind. We 

 have seen that the eggs of the shads are free and develop at the 

 bottom of the water, while the eggs of the sprat and pilchard are 



