23 



The fact that mature or nearly mature fish eggs may 

 float at the surface is not conclusive proof that they have 

 been fertilised. From a number of experiments made at 

 Piel during the past season it was found that unfertilised 

 plaice eggs would remain floating even up to five days 

 after being emitted by the fish. To the unaided eye these 

 eggs looked healthy enough, but when submitted to 

 microscopic examination, were at once seen to be dead. 

 Therefore, although nine millions of floating eggs were 

 landed at Piel, probably less than half were in a suitable 

 condition for incubating. Indeed, it must be evident that 

 a very large proportion of the eggs belonging to the pelagic 

 group, which are obtained by pressing the sides of the fish, 

 are not mature enough to be fertilised. From previous 

 observations it is known that a female plaice may take at 

 least two weeks to discharge the whole contents of her 

 ovaries. In nature only a small proportion of the eggs are 

 emitted at a time. Eggs that naturally incubate on the 

 bottom of the sea, on the other hand, may be entirely 

 deposited in a day. Much of the above-noted very con- 

 siderable loss would be avoided if we had a suitable open- 

 air pond where we could keep mature fish and allow them 

 to spawn of their own accord. We have already tried this 

 on a small scale, with satisfactory results, but our present 

 indoor tanks are much too small to accommodate a 

 sufdcient number of fish to produce enough eggs to fill 

 the hatching boxes. 



The present capacity of the whole hatching apparatus 

 may be conveniently stated by representing it in plaice 

 eggs. The number of these that can be incubated at one 

 time is 23 millions. Under favourable circumstances two 

 such quantities could be dealt with in one season. To 

 put it in another way : — a mature female plaice produces, 

 on an average, 300,000 eggs, it would therefore take 75 



