We must observe how rightly Nature orders generation in regular
gradation. The more perfect and hotter animals produce their young
perfect in respect of quality (in respect of quantity this is so with
no animal, for the young always increase in size after birth), and
these generate living animals within themselves from the first. The
second class do not generate perfect animals within themselves from
the first (for they are only viviparous after first laying eggs),
but still they are externally viviparous. The third class do not
produce a perfect animal, but an egg, and this egg is perfect. Those
whose nature is still colder than these produce an egg, but an
imperfect one, which is perfected outside the body, as the class of
scaly fishes, the crustacea, and the cephalopods. The fifth and
coldest class does not even lay an egg from itself; but so far as
the young ever attain to this condition at all, it is outside the body
of the parent, as has been said already. For insects produce a
scolex first; the scolex after developing becomes egg-like (for the
so-called chrysalis or pupa is equivalent to an egg); then from
this it is that a perfect animal comes into being, reaching the end of
its development in the second change.
Some animals then, as said before, do not come into being from
semen, but all the sanguinea do so which are generated by
copulation, the male emitting semen into the female when this has
entered into her the young are formed and assume their peculiar
character, some within the animals themselves when they are
viviparous, others in eggs.
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