Heavy, adhesive eggs, are received upon bunches of twigs, or frames of glass plates, to which they adhere, and which are placed in receptacles through which water is passing.
Semi-buoyant eggs, or those whose specific gravity is but slightly greater than that of the water, require altogether other treatment. They are necessarily placed together in large numbers, and to prevent their settling upon the bottom of the receptacle, it is necessary to introduce a gentle current from below. For many years these eggs could be hatched only in floating receptacles placed in a river, with wire cloth bottoms, placed at an angle, the motion of which was utilized to keep the eggs in suspension. Later, an arrangement of plunging buckets was invented, cylindrical receptacles with tops and bottoms of wire cloth, which were worked up and down at the surface of the water by machinery. The eggs in the cylinders were suspended in rows from beams which were worked up and down at the surface of the water by machinery. The eggs in the cylinders were thus kept constantly in motion. Finally, the device now most in favor was perfected; this is a receptacle, conical, or at least with a constricted termination, placed with its apex downward, through which passes from below a strong current, keeping the eggs constantly suspended and in motion. This form of apparatus, of which the McDonald and Clark hatching jars are the most perfect developments, may be worked in connection with any common hydrant.[A]
Floating eggs have been hatched only by means of rude contrivances for sustaining a lateral circular eddy, or swirl of water in the receptacle.
The use of refrigerators, to retard the development of the egg until such time as it is most convenient to take care of the fry, is now extensively practiced in the United States, and has been experimented upon in Germany.
In the discussion of fish-cultural economy, the distinction between _private fish culture_ and _public fish culture_ must be carefully observed, and it must also be borne in mind that by _public fish culture_, or _modern fish culture_, I mean fish culture carried on at public expense, and for the public good. Public fish culture, to be effective, must be conducted by men trained in scientific methods of thought and work.