154 How to obtain it. 
judicious crossing of races and varieties is an absolute necessity if 
successful fish farming is to be carried on. An accepted theory 
amongst viologists is that though the reproductive cells as a rule. 
vary considerably in size between one genus and another, it is less 
marked in the species, and disappears (especially in the male 
element) in the varieties. 
An expert can often tell at a glance whether a fish is ripe or 
not. There are signs which it is difficult to describe, such as the 
appearance of the organs, the looseness of the ova in the fish, and 
a general appearance of ripeness, which can only be detected by 
long practice. No force should be used in expelling the eggs. If 
they do not flow freely on the hand being very gently passed over 
the belly of the fish they are not ripe. Of course a slight pressure 
is necessary, but far more depends upon proper handling. The 
hand or thumb should be kept behind the eggs, and not allowed 
to run over or in front of them. In taking the milt a knowledge 
of the situation of the glands is useful. They are much lower 
down than the ovaries of the female, and instead of using the 
hand the milt may be easily obtained by an adroit use of the 
thumb and forefinger. The weather has much influence on the 
spawning of fish. A mild day, preceded by a warm rain, will 
make the fish lively, and they will spawn freely ; but a hard frost, 
or snow in the water, will retard the shedding of the ova, and > 
consequently, on a fish farm, preparations are often made accord- 
‘ingly. We know, as a rule, before commencing, how the fish will 
yield their ova. 
When the eggs have had the milt on them long enough, they 
are easily washed in the spawning dishes by pouring off a part of 
the water, adding more that is fresh and clean, and continuing the 
Operation some six or seven times, when the eggs should be clean. 
Be quite sure that they are so, and should there be any doubt 
about it give them another wash. At the Solway Fishery, where 
eggs are dealt with in large quantities, the contents of a dozen or 
a score of spawning dishes are emptied into a well-charred wooden 
tank, through which a current of water flows, and are left there for 
half-an-hour or more, when they are found to be perfectly clean. 
(Fig. 13). 
One of these tanks will wash a hundred thousand eggs at a 
