SEA-FISHERIES LABORATORY. 167 
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 
eges 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 
sufficient 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 
egos. ‘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 
