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On The Generation Of Animals   

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. This is what we find in
lake birds, for they are of a moister nature and are colder than the
land birds, so that the so-called 'lecithus' or yolk in the eggs of
such birds is large and less yellow because the white is less
separated off from it. But when we come to the ovipara which are
both of a cold nature and also moister (such is the fish kind) we
find the white not separated at all because of the small size of the
eggs and the quantity of the cold and earthy matter; therefore all
fish eggs are of one colour, and white compared with yellow, yellow
compared with white. Even the wind-eggs of birds have this distinction
of colour, for they contain that out of which will come each of the
two parts, alike that whence arises the principle of life and that
whence comes the nutriment; only both these are imperfect and need the
influence of the male in addition; for wind-eggs become fertile if
impregnated by the male within a certain period. The difference in
colour, however, is not due to any difference of sex, as if the
white came from the male, the yolk from the female; both on the
contrary come from the female, but the one is cold, the other hot.
In all cases then where the hot part is considerable it is separated
off, but where it is little it cannot be so; hence the eggs of such
animals, as has been said, are of one colour. The semen of the male
only puts them into form; and therefore at first the egg in birds
appears white and small, but as it advances it is all yellow as more
of the sanguineous material is continually mixed with it; finally as
the hot part is separated the white takes up a position all round it
and equally distributed on all sides, as when a liquid boils; for
the white is naturally liquid and contains in itself the vital heat;
therefore it is separated off all round, but the yellow and earthy
part is inside. And if we enclose many eggs together in a bladder or
something of the kind and boil them over a fire so as not to make
the movement of the heat quicker than the separation of the white
and yolk in the eggs, then the same process takes place in the whole
mass of the eggs as in a single egg, all the yellow part coming into
the middle and the white surrounding it.

We have thus stated why some eggs are of one colour and others of


The principle of the male is separated off in eggs at the point
where the egg is attached to the uterus, and the reason why the
shape of two-coloured eggs is unsymmetrical, and not perfectly round
but sharper at one end, is that the part of the white in which is
contained this principle must differ from the rest. Therefore the

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