FISH-HATCHING AT THE FISHERIES EXHIBITION. 261 
FISH-HATCHING AT THE FISHERIES 
EXHIBITION. 
By W. BLAcKBURN, F.R.MLS. 
MONGST the most interesting features of the Exhibition are 
the various apparatus for hatching and rearing fish. Here 
fish may be seen in all stages of development; the ovum without 
any apparent signs of life, or in some instances with two black 
spots just visible to the naked eye, indicative of the growing organs 
of vision of the inclosed embryo; the newly hatched fry, with the 
umbilical vesicle attached to it and affording it a temporary supply 
of food until it becomes capable, by further development, of main- 
taining the struggle for existence by the exercise of its senses ; and 
the perfect fish, with trained eye and muscle, ready to make the 
best of every condition of its environment. 
The microscopist who takes especial interest in the early stages 
of the fish development is naturally desirous of possessing some 
simple means of keeping the ova in a healthy state for observation 
and experiment; and this, in the case of ova that do not float, 
can be secured only by a constant supply of water running over 
them. 
The simplest contrivance for this purpose appears to me to be 
* McDonald’s Universal Hatching-Jar,” patented by Mr. Marshall 
McDonald of Washington, U.S.A. It consists of a glass jar, 15 
inches high and 6 inches in diameter, to hold five quarts of water ; 
and it can be filled two-thirds full of eggs with safety. 60,000 ova 
of the shad are not too many to place in it at one time. It is 
covered by a cap of metal, through the centre of which a glass tube, 
for the inflow of water, passes nearly to the bottom of the jar. 
Another glass tube, shorter than the former, passes through the cap 
near the side of the jar, and serves as an outflow pipe. ‘This pipe, 
the lower end of which is usually some distance above the eggs, 
can be lowered at discretion so as to be brought into contact with 
any dead eggs, which, by reason of their comparative lightness, may 
have risen above the rest. Dead or decaying eggs are thus carried 
away, and fungoid growth is prevented. The inflow of water, 
striking the bottom of the jar, is deflected upwards around its 
sides, carrying the eggs with it; these then move towards the 
centre of the jar, and fall until they come again into contact with 
the upward current. This current must be regulated to give suit- 
able buoyancy to the eggs. The heavier kinds, like those of the 
salmon, do not move with the current, but are merely relieved 
from the pressure of those above them. 
