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important. Much capital has been made by the trawlers that 
they do not injure sea fisheries because cod eggs float. But 
it has been kept as much as possible in the dark that they only 
do so under certain conditions of the water in which they are 
exuded. If a proportion of fresh water becomes mixed with 
that of the sea so as to alter its usual specific gravity, then the 
eggs of cod fishes sink. The United States Expedition have 
found hake with ova, and the young at from 100 to 200 fathoms 
depth. Some marine fishes’ eggs, as that of flat-fish, or 
Pluronectide, are recorded to float so long as the water is 
agitated, but to subside when it is at rest. 
Some fishes are sterile from various causes. Thus the com- 
mon eel, a catadromous form, or one which breeds in the sea 
but passes its life in fresh waters, is believed only to deposit 
ova once during its lifetime, and then either dies or returns to 
the rivers, and is sterile for the remainder of its life. It has 
been observed that among the Salmonide sterile forms are seen, 
but which are believed to be only temporarily so, as for one of 
two seasons, while, as far as I have had the opportunity of 
_ observing, hybrids between species of Salmonide are sterile. 
Fish may also be sterile due to disease. Thus I have seen 
in a mackerel the oviduct occluded due to disease having set 
q up inflammation and occlusion of the outlet of the oviduct, and 
_ thus the preceding years eggs have been retained and formed a 
i large tumour. 
Eggs, themselves, of course, may fail in hatching, due to 
- deficiency of fecundation, injurious surroundings, or consequent 
_ upon the effects of disease, or the eggs, (as deposited by some 
hybrids) may be incapable of fertilisation. Fishes’ eggs are 
more or less circular or oval, and of varying colours, being pea 
_ green in some sheat fishes or siluroids, while among the Sal- 
—monide they may be coral-red, yellow, or pure white, the 
herrings usually have a slight pink tinge, while those of the 
-sprat are colourless. 
T have already remarked that fishes’ eggs before they are 
fertilised have a small orifice or micropyle into which the 
Spermatozoon enters, but it is evident. in sea fishes that if the 
