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On The Generation Of Animals   

it, attracts the semen. The discharge and collection of the
catamenia also excite heat in this part. Hence it acts like
cone-shaped vessels which, when they have been washed out with hot
water, their mouth being turned downwards, draw water into themselves.
And this is the way things are drawn up, but some say that nothing
of the kind happens with the organic parts concerned in copulation.
Precisely the opposite is the case of those who say the woman emits
semen as well as the man, for if she emits it outside the uterus
this must then draw it back again into itself if it is to be mixed
with the semen of the male. But this is a superfluous proceeding,
and Nature does nothing superfluous.

When the material secreted by the female in the uterus has been
fixed by the semen of the male (this acts in the same way as rennet
acts upon milk, for rennet is a kind of milk containing vital heat,
which brings into one mass and fixes the similar material, and the
relation of the semen to the catamenia is the same, milk and the
catamenia being of the same nature)- when, I say, the more solid
part comes together, the liquid is separated off from it, and as the
earthy parts solidify membranes form all round it; this is both a
necessary result and for a final cause, the former because the surface
of a mass must solidify on heating as well as on cooling, the latter
because the foetus must not be in a liquid but be separated from it.
Some of these are called membranes and others choria, the difference
being one of more or less, and they exist in ovipara and vivipara

When the embryo is once formed, it acts like the seeds of plants.
For seeds also contain the first principle of growth in themselves,
and when this (which previously exists in them only potentially) has
been differentiated, the shoot and the root are sent off from it,
and it is by the root that the plant gets nourishment; for it needs
growth. So also in the embryo all the parts exist potentially in a way
at the same time, but the first principle is furthest on the road to
realization. Therefore the heart is first differentiated in actuality.
This is clear not only to the senses (for it is so) but also on
theoretical grounds. For whenever the young animal has been
separated from both parents it must be able to manage itself, like a
son who has set up house away from his father. Hence it must have a
first principle from which comes the ordering of the body at a later
stage also, for if it is to come in from outside at later period to
dwell in it, not only may the question be asked at what time it is
to do so, but also we may object that, when each of the parts is
separating from the rest, it is necessary that this principle should
exist first from which comes growth and movement to the other parts.

(Wherefore all who say, as did Democritus, that the external parts of
animals are first differentiated and the internal later, are much
mistaken; it is as if they were talking of animals of stone or wood.
For such as these have no principle of growth at all, but all
animals have, and have it within themselves.) Therefore it is that
the heart appears first distinctly marked off in all the sanguinea,
for this is the first principle or origin of both homogeneous and
heterogeneous parts, since from the moment that the animal or organism
needs nourishment, from that moment does this deserve to be called its
principle or origin. For the animal grows, and the nutriment, in its
final stage, of an animal is the blood or its analogue, and of this
the blood-vessels are the receptacle, wherefore the heart is the
principle or origin of these also. (This is clear from the
Enquiries and the anatomical drawings.)

Since the embryo is already potentially an animal but an imperfect
one, it must obtain its nourishment from elsewhere; accordingly it
makes use of the uterus and the mother, as a plant does of the

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