INCUBATION. 97 



The membrane of the sac is scarcely thicker than 

 a spider' s web, and as easily ruptured. With the 

 fishes will be carried by the water into the pan 

 egg shells and other dirt. By careful manipula- 

 tion the lighter dirt may be floated off, and the re- 

 mainder must be picked out with the bulb syringe. 

 Remember that three weeks at least must elapse 

 before the gravel upon which the young are placed 

 can be washed, and that every particle of dirt 

 must be scrupulously removed. If no nursery 

 trough has been provided, the fishes may be 

 reared for some time in the tray. For this the 

 grille is removed, the box thoroughly washed, and 

 the bottom covered with fine gravel not only 

 washed, but boiled, that all larvae of insects may 

 be destroyed. Upon this should the troutlets be 

 placed, and a gentle current of water allowed to 

 flow over them. Should, however, nursery -troughs 

 have been provided, they may be prepared in the 

 same manner, taking care that the wood work be 

 not only washed, but scalded. Our nursery- 

 troughs are thirty feet long, fourteen inches wide, 

 and three deep. By screens of fine copper wire 

 gauze, or iron well coated with asphalt varnish, 

 they are divided into compartments one foot 

 in length, each compartment being of sufficient 

 capacity for the accommodation of fifteen hun- 

 dred fishes, though one thousand only are usually 



5 



