We have now spoken of the production of few and many young, and of
the outgrowth of superfluous parts or of their deficiency, and also of
monstrosities.
5
Superfoetation does not occur at all in some animals but does in
others; of the former some are able to bring the later formed embryo
to birth, while others can only do so sometimes. The reason why it
does not occur in some is that they produce only one young one, for it
is not found in solid-hoofed animals and those larger than these, as
owing to their size the secretion of the female is all used up for the
one embryo. For all these have large bodies, and when an animal is
large its foetus is large in proportion, e.g. the foetus of the
elephant is as big as a calf. But superfoetation occurs in those which
produce many young because the production of more than one at a
birth is itself a sort of superfoetation, one being added to
another. Of these all that are large, as man, bring to birth the later
embryo, if the second impregnation takes place soon after the first,
for such an event has been observed before now. The reason is that
given above, for even in a single act of intercourse the semen
discharged is more than enough for one embryo, and this being
divided causes more than one child to be born, the one of which is
later than the other. But when the embryo has already grown to some
size and it so happens that copulation occurs again, superfoetation
sometimes takes place, but rarely, since the uterus generally closes
in women during the period of gestation.
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