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
earth, to get nourishment, until it is perfected to the point of being
now an animal potentially locomotive. So Nature has first designed the
two blood-vessels from the heart, and from these smaller vessels
branch off to the uterus. These are what is called the umbilicus,
for this is a blood-vessel, consisting of one or more vessels in
different animals. Round these is a skin-like integument, because
the weakness of the vessels needs protection and shelter. The
vessels join on to the uterus like the roots of plants, and through
them the embryo receives its nourishment. This is why the animal
remains in the uterus, not, as Democritus says, that the parts of
the embryo may be moulded in conformity with those of the mother. This
is plain in the ovipara, for they have their parts differentiated in
the egg after separation from the matrix.
Here a difficulty may be raised. If the blood is the nourishment,
and if the heart, which first comes into being, already contains
blood, and the nourishment comes from outside, whence did the first
nourishment enter? Perhaps it is not true that all of it comes from
outside just as in the seeds of plants there is something of this
nature, the substance which at first appears milky, so also in the
material of the animal embryo the superfluous matter of which it is
formed is its nourishment from the first.
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