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On The Generation Of Animals   



But the cartilaginous fish and the vipers produce their young
alive externally, but first produce eggs internally. The egg is
perfect, for so only can an animal be generated from an egg, and
nothing comes from an imperfect one. It is because they are of a
cold nature, not hot as some assert, that they do not lay their eggs
externally.

11

At least they certainly produce their eggs in a soft envelope, the
reason being that they have but little heat and so their nature does
not complete the process of drying the egg-shell. Because, then,
they are cold they produce soft-shelled eggs, and because the eggs are
soft they do not produce them externally; for that would have caused
their destruction.

The process is for the most part the same as in birds, for the egg
descends and the young is hatched from it near the vagina, where the
young is produced in those animals which are viviparous from the
beginning. Therefore in such animals the uterus is dissimilar to
that of both the vivipara and ovipara, because they participate in
both classes; for it is at once near the hypozoma and also
stretching along downwards in all the cartilaginous fishes. But the
facts about this and the other kinds of uterus must be gathered from
inspection of the drawings of dissections and from the Enquiries.
Thus, because they are oviparous, laying perfect eggs, they have the
uterus placed high, but, as being viviparous, low, participating in
both classes.

Animals that are viviparous from the beginning all have it low,
Nature here having no other business to interfere with her, and
their production having no double character. Besides this, it is
impossible for animals to be produced alive near the hypozoma, for the
foetus must needs be heavy and move, and that region in the mother
is vital and would not be able to bear the weight and the movement.
Thirdly, parturition would be difficult because of the length of the
passage to be traversed; even as it is there is difficulty with
women if they draw up the uterus in parturition by yawning or anything
of the kind, and even when empty it causes a feeling of suffocation if
moved upwards. For if a uterus is to hold a living animal it must be
stronger than in ovipara, and therefore in all the vivipara it is
fleshy, whereas when the uterus is near the hypozoma it is membranous.
And this is clear also in the case of the animals which produce
young by the mixed method, for their eggs are high up and sideways,
but the living young are produced in the lower part of the uterus.

So much for the reason why differences are found in the uterus of
various animals, and generally why it is low in some and high in
others near the hypozoma.

12

Why is the uterus always internal, but the testes sometimes
internal, sometimes external? The reason for the uterus always being
internal is that in this is contained the egg or foetus, which needs
guarding, shelter, and maturation by concoction, while the outer
surface of the body is easily injured and cold. The testes vary in
position because they also need shelter and a covering to preserve
them and to mature the semen; for it would be impossible for them,
if chilled and stiffened, to be drawn up and discharge it.
Therefore, whenever the testes are visible, they have a cuticular
covering known as the scrotum. If the nature of the skin is opposed to
this, being too hard to be adapted for enclosing them or for being

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