Among creatures that lay eggs externally birds produce their egg
perfect, fish imperfect, but the eggs of the latter complete their
growth outside as has been said before. The reason is that the fish
kind is very fertile; now it is impossible for many eggs to reach
completion within the mother and therefore they lay them outside. They
are quickly discharged, for the uterus of externally oviparous
fishes is near the generative passage. While the eggs of birds are
two-coloured, those of all fish are one-coloured. The cause of the
double colour may be seen from considering the power of each of the
two parts, the white and the yolk. For the matter of the egg is
secreted from the blood [No bloodless animal lays eggs,] and that
the blood is the material of the body has been often said already. The
one part, then, of the egg is nearer the form of the animal coming
into being, that is the hot part; the more earthy part gives the
substance of the body and is further removed. Hence in all
two-coloured eggs the animal receives the first principle of
generation from the white (for the vital principle is in that which
is hot), but the nutriment from the yolk. Now in animals of a
hotter nature the part from which the first principle arises is
separated off from the part from which comes the nutriment, the one
being white and the other yellow, and the white and pure is always
more than the yellow and earthy; but in the moister and less hot the
yolk is more in quantity and more fluid.
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