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On The Generation Of Animals   

egg is harder at this point than below, for it is necessary to shelter
and protect this principle. And this is why the sharp end of the egg
comes out of the hen later than the blunt end; for the part attached
to the uterus comes out later, and the egg is attached at the point
where is the said principle, and the principle is in the sharp end.
The same is the case also in the seeds of plants; the principle of the
seed is attached sometimes to the twig, sometimes to the husk,
sometimes to the pericarp. This is plain in the leguminous plants, for
where the two cotyledons of beans and of similar seeds are united,
there is the seed attached to the parent plant, and there is the
principle of the seed.

A difficulty may be raised about the growth of the egg; how is it
derived from the uterus? For if animals derive their nutriment through
the umbilical cord, through what do eggs derive it? They do not,
like a scolex, acquire their growth by their own means. If there is
anything by which they are attached to the uterus, what becomes of
this when the egg is perfected? It does not come out with the egg as
the cord does with animals; for when its egg is perfected the shell
forms all round it. This problem is rightly raised, but it is not
observed that the shell is at first only a soft membrane, and that
it is only after the egg is perfected that it becomes hard and
brittle; this is so nicely adjusted that it is still soft when it
comes out (for otherwise it would cause pain in laying), but no
sooner has it come out than it is fixed hard by cooling, the
moisture quickly evaporating because there is but little of it, and
the earthy part remaining. Now at first a certain part of this
membrane at the sharp end of eggs resembles an umbilical cord, and
projects like a pipe from them while they are still small. It is
plainly visible in small aborted eggs, for if the bird be drenched
with water or suddenly chilled in any other way and cast out the egg
too soon, it appears still sanguineous and with a small tail like an
umbilical cord running through it. As the egg becomes larger this is
more twisted round and becomes smaller, and when the egg is
perfected this end is the sharp end. Under this is the inner
membrane which separates the white and the yolk from this. When the
egg is perfected, the whole of it is set free, and naturally the
umbilical cord does not appear, for it is now the extreme end of the
egg itself.

The egg is discharged in the opposite way from the young of
vivipara; the latter are born head-first, the part where is the
first principle leading, but the egg is discharged as it were feet
first; the reason of this being what has been stated, that the egg
is attached to the uterus at the point where is the first principle.

The young bird is produced out of the egg by the mother's incubating
and aiding the concoction, the creature developing out of part of
the egg, and receiving growth and completion from the remaining
part. For Nature not only places the material of the creature in the
egg but also the nourishment sufficient for its growth; for since
the mother bird cannot perfect her young within herself she produces
the nourishment in the egg along with it. Whereas the nourishment,
what is called milk, is produced for the young of vivipara in
another part, in the breasts, Nature does this for birds in the egg.
The opposite, however, is the case to what people think and what is
asserted by Alcmaeon of Crotona. For it is not the white that is the
milk, but the yolk, for it is this that is the nourishment of the
chick, whereas they think it is the white because of the similarity of

The chick then, as has been said, comes into being by the incubation
of the mother; yet if the temperature of the season is favourable,
or if the place in which the eggs happen to lie is warm, the eggs

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