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On The Generation Of Animals   

generative secretion is failing along with the advance of years.

We have now stated in which birds wind-eggs are found, and also what
sort of birds lay many eggs or few, and for what reasons. And
wind-eggs, as said before, come into being because while it is the
material for generation that exists in the female of all animals,
birds have no discharge of catamenia like viviparous sanguinea (for
they occur in all these latter, more in some, less in others, and in
some only enough in quantity just to mark the class). The same
applies to fish as to birds, and so in them as in birds is found an
embryonic formation without impregnation, but it is less obvious
because their nature is colder. The secretion corresponding to the
catamenia of vivipara is formed in birds at the appropriate season for
the discharge of superfluous matter, and, because the region near
the hypozoma is hot, it is perfected so far as size is concerned,
but in birds and fishes alike it is imperfect for generation without
the seminal fluid of the male; the cause of this has been previously
given. Wind-eggs are not formed in the flying birds, for the same
reason as prevents their laying many eggs; for the residual matter
in birds of prey is small, and they need the male to give an impulse
for the discharge of it. The wind-eggs are produced in greater numbers
than the impregnated but smaller in size for one and the same
reason; they are smaller in size because they are imperfect, and
because they are smaller in size they are more in number. They are
less pleasant for food because they are less concocted, for in all
foods the concocted is more agreeable. It has been sufficiently
observed, then, that neither birds' nor fishes' eggs are perfected for
generation without the males. As for embryos being formed in fish also

(though in a less degree) without the males, the fact has been
observed especially in river fish, for some are seen to have eggs from
the first, as has been written in the Enquiries concerning them. And
generally speaking in the case of birds even the impregnated eggs
are not wont for the most part to attain their full growth unless
the hen be trodden continually. The reason of this is that just as
with women intercourse with men draws down the secretion of the
catamenia (for the uterus being heated attracts the moisture and
the passages are opened), so this happens also with birds; the
residual matter corresponding to the catamenia advances a little at
a time, and is not discharged externally, because its amount is
small and the uterus is high up by the hypozoma, but trickles together
into the uterus itself. For as the embryo of the vivipara grows by
means of the umbilical cord, so the egg grows through this matter
flowing to it through the uterus. For when once the hens have been
trodden, they all continue to have eggs almost without intermission,
though very small ones. Hence some are wont to speak of wind-eggs as
not coming into being independently but as mere relics from a previous
impregnation. But this is a false view, for sufficient observations
have been made of their arising without impregnation in chickens and
goslings. Also the female partridges which are taken out to act as
decoys, whether they have ever been impregnated or not, immediately on
smelling the male and hearing his call, become filled with eggs in the
latter case and lay them in the former. The reason why this happens is
the same as in men and quadrupeds, for if their bodies chance to be in
rut they emit semen at the mere sight of the female or at a slight
touch. And such birds are of a lascivious and fertile nature, so
that the impulse they need is but small when they are in this
excited condition, and the secreting activity takes place quickly in
them, wind-eggs forming in the unimpregnated and the eggs in those
which have been impregnated growing and reaching perfection swiftly.

Among creatures that lay eggs externally birds produce their egg
perfect, fish imperfect, but the eggs of the latter complete their
growth outside as has been said before. The reason is that the fish

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