128 DOMESTICATED TROUT. 



tion of previously removed eggs to the number of 

 empty ones, and you get at the proportion of impreg- 

 nated eggs. This, however, only answers for the par- 

 ticular box from which these were taken. To obtain 

 the percentage of the whole season's yield, this opera- 

 tion must be repeated with each box or compartment. 

 It will be well to observe here, also, that it is a good 

 plan, as soon as the impregnated eggs are unmistaka- 

 bly distinguishable from the empty ones, to take them 

 all but into pans, and remove all the empty ones 

 before replacing them in the hatching-boxes. The 

 work of picking over will be done much easier and 

 quicker this way, and it has this great advantage, 

 that it is done once for all, and you are for the rest 

 of the season relieved of the burden of care which 

 the daily necessity of removing the empty ones in- 

 volves. 



The time required for hatching depends chiefly on 

 the temperature of the water. Seth Green's rule is 

 that at 50° Fahrenheit trout eggs will hatch out in 

 fifty days, and every degree warmer or colder makes 

 five days' difference in time ; warmer water shortening 

 the period, and colder water lengthening it. Green 

 also says, that if the fish are hatched in fifty days, the 

 yolk sac remains thirty more. If in seventy days, the 

 sac remains forty-five days. 



