504 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [14] 
preferable to bave hotching-canals, which should have solid walls, so 
as to exclude water-rats, as well as the Disticus marginalis and Hy- 
drophilus piceus (both as larve and in their perfect state), and a good 
covering, and be provided with wire gratings, both to protect the fish 
from the voracity of other inhabitants of the water and to prevent their 
dispersion. . 
These coverings also prevent the appearance of the greenish mold 
which generally forms on the stones at the bottom, and which is com- 
posed of diatoms and other small alge. 
CHAPTER III. 
§ 1.—Hatching eggs in incubating apparatus. 
After the eggs have been fecundated either near or in water having 
the same temperature as that in which the hatching apparatus is placed, 
they are carefully put in this apparatus. By observing these condi- 
tions no disturbance need be feared from any sudden change in the 
temperature of the water. For incubating the free eggs of salmon and 
other winter fish, the specific weight of which is much greater than that 
of the water, and which consequently descend to the bottom of the 
apparatus, it will be necessary to furnish the apparatus with a layer of 
gravel several centimeters thick, and to see to it that this layer is uni- 
formly covered with the eggs. Mr. Detzem and several other practical 
pisciculturists put a layer of fine sand on these eggs; others again avoid 
doing this, so as to be able to constantly watch the eggs, and to remove 
any spoiled eggs or causes of destruction. 
As regards those eggs which adhere to foreign bodies, like those of 
the carp, &c., which are lighter than the water in which they float, it is 
necessary to place them in the apparatus with the plants on which they 
have been deposited, as has been described in Chapter I, § 3, of this 
treatise. It is necessary to avoid currents which would carry the eggs 
to one single part of the apparatus. It will, therefore, be best to select 
the stagnant and tranquil waters of ponds or canals where the effect of 
strong currents can be mitigated by using apparatus with very close 
wire gratings. In thiscase the apparatus must not be entirely submerged 
but should be placed in such a position that there is an empty space 
’ between the water and the lid. A few centimeters of water suffice for 
apparatus in which the water is easily and regularly renewed. 
It should be mentioned that with Coste’s apparatus and the one 
used in Holland, the eggs are deposited on glass or wire frames, and 
that no gravel is required; whilst the ingenious system of artificial brooks 
with constantly running water, invented by Coste,* regulates the dis- 
tribution of the water in the most suitable manner. 
It should also be observed that eggs which have been transported, 
or which come from a distance, should be gradually accustomed to the 
* Comptes-Rendus, 1852, p. 301. — 
