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and the herrings may permanently, or at least for an indefinite 
number of years, migrate to a more suitable spot. The sprat 
gives eggs of about 0:04 in. in diameter, and these likewise 
sink to the bottom, but not in a mass or covered with adhering 
substance, as in the herring. The shad of our waters has not 
yet had this question investigated, but in the United States the 
Fish Commission find that in the species which is most prized 
there, Clupea sapidissima, the eggs sink, but that they require 
to be kept in constant motion. Placed in floating boxes, it was 
observed that they experienced this when a strong current 
existed, but during slack tides the motion was insufficient, they 
rested in masses at the bottom of the box, and being very 
liable to fungus, especially during high temperatures, they 
became diseased unless stirred up by the hand. Without 
entering into the composition of the apparatus employed, it 
was found that for their successful hatching constant motion 
is necessary. Thus in one genus are forms in which the 
eges are agglutinated into a mass, sink, and become attached 
_ to suitable fixed objects, in another they simply sink, whereas 
in a third, although they sink, they require to be kept in 
constant motion. 
If we take another family for investigation, as that of the 
salmon, trout, and their allies, we find interesting variations. 
The common smelt (Osmerus eperlanus) covers stones, planks, 
and suitable objects with its adherent ova, which are placed 
near the level of high water, for the purpose of adhesion. 
These eggs are furnished with fine filaments on their outer 
_ surface, which filaments expand at their distal extremities into 
_ the form of a sucker for attachment. The grayling deposits 
_ its ova, about April or May, or even earlier, on the gravel at 
_ the bottom of a suitable stream; they are not placed in a nest, 
_ and appear to be very delicate; their size is rather less than 
seen in the trout. But the salmon, trout, and charr fan up the 
_ gravel, thus forming a trough wherein the ova are deposited, 
_ and subsequently the nest, redd or rid, is covered over with the 
_ gravel, and here the eggs are left to come to maturity. In all 
_ these forms the eggs are heavier than the water in which they 
