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On The Generation Of Animals   

Other animals are born in possession of teeth or their analogue
(unless in cases contrary to Nature), because when they are set
free from the parent they are more perfect than man; but man (also
unless in cases contrary to Nature) is born without them.

The reason will be stated later why some teeth are formed and fall
out but others do not fall out.

It is because such parts are formed from a residue that man is the
most naked in body of all animals and has the smallest nails in
proportion to his size; he has the least amount of earthy residue, but
that part of the blood which is not concocted is the residue, and
the earthy part in the bodies of all animals is the least concocted.
We have now stated how each of the parts is formed and what is the
cause of their generation.


In viviparous animals, as said before, the embryo gets its growth
through the umbilical cord. For since the nutritive power of the soul,
as well as the others, is present in animals, it straightway sends off
this cord like a root to the uterus. The cord consists of
blood-vessels in a sheath, more numerous in the larger animals as
cattle and the like, one in the smallest, two in those of intermediate
size. Through this cord the embryo receives its nourishment in the
form of blood, for the uterus is the termination of many
blood-vessels. All animals with no front teeth in the upper jaw, and
all those which have them in both jaws and whose uterus has not one
great blood-vessel running through it but many close together instead-
all these have in the uterus the so-called cotyledons (with which the
umbilical cord connects and is closely united; for the vessels which
pass through the cord run backwards and forwards between embryo and
uterus and split up into smaller vessels all over the uterus; where
they terminate, there are found the cotyledons). Their convexity is
turned towards the uterus, the concavity towards the embryo. Between
uterus and embryo are the chorion and the membranes. As the embryo
grows and approaches perfection the cotyledons become smaller and
finally disappear when it is perfected. For Nature sends the
sanguineous nutriment for the embryo into this part of the uterus as
she sends milk into the breasts, and because the cotyledons are
gradually aggregated from many into a few the body of the cotyledon
becomes like an eruption or inflammation. So long as the embryo is
comparatively small, being unable to receive much nutriment, they
are plain and large, but when it has increased in size they fall in

But most of the animals which have front teeth in both jaws and no
horns have no cotyledons in the uterus, but the umbilical cord runs to
meet one blood-vessel, which is large and extends throughout the
uterus. Of such animals some produce one young at a time, some more
than one, but the same description applies to both these classes.
(This should be studied with the aid of the examples drawn in the
Anatomy and the Enquiries.) For the young, if numerous, are
attached each to its umbilical cord, and this to the blood-vessel of
the mother; they are arranged next to one another along the stream
of the blood-vessel as along a canal; and each embryo is enclosed in
its membranes and chorion.

Those who say that children are nourished in the uterus by sucking
some lump of flesh or other are mistaken. If so, the same would have
been the case with other animals, but as it is we do not find this
(and this can easily be observed by dissection). Secondly, all
embryos alike, whether of creatures that fly or swim or walk, are

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