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egg floats at the surface and the milt is beneath, the chances 
of fertilisation must be diminished, unless some means to 
obviate this are in existence. Also that there must exist some 
mechanical reason for fish eggs to float in some forms, sink in 
others. Of course, the principal cause which makes eggs sub- 
side to the bottom is that their specific gravity is greater than 
that of the fluid in which they are floating, unless due to some 
mechanical arrangement (as the presence of filaments) they are 
attached to foreign substances, when they would sink or swim 
in accordance with the condition of the body to which they 
were attached, as the eggs of the marine gar fish; ora fish (as 
a perch) may have its ova in a band-like state, when it selects 
rushes, reeds, or grass growing in the water or a piece of wood 
or other hard substance, against which it (the female) presses 
itself until one end of the band has become attached, then 
swimming slowly away the eggs are voided. But sometimes 
eggs, as of the cod, float in normally saline water, and questions 
have arisen as to the position of the micropyle. Dr Ransom in 
1854 found that in the trout, salmon, and grayling it corres- 
ponded to the centre of the germinal pole. Here the formative 
yelk or germ collects, and having attached to it some oil drops, 
always floats uppermost. In the Spanish mackerel and some 
other American forms a single Jarge oil sphere keeps them 
buoyant, situated at a point immediately opposite the germinal 
disk, which is constantly inverted or carried on the lower face 
of the vitellus, thus acting exactly the reverse as observed 
among the Salmonide. Inthe cod no oil drop exists, but the 
egg is so light that it behaves like the foregoing. It is seen in 
the cod fisheries that at the period of breeding the egg floats 
with the micropyle directed downwards, and as a consequence 
the milters are found to swim lower than the spawners, the 
milt must consequently ascend. The reverse is observed in the 
Salmonide. 
It will now be necessary to briefly remark upon the physical 
changes which fishes’ eggs have to undergo prior to their being 
rendered in a suitable condition to continue the species. If 
we examine the ovum of an osseous fish under the microscope 
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