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.
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