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On The Generation Of Animals   

are sufficiently concocted without incubation, both those of birds and
those of oviparous quadrupeds. For these all lay their eggs upon the
ground, where they are concocted by the heat in the earth. Such
oviparous quadrupeds as do visit their eggs and incubate do so
rather for the sake of protecting them than of incubation.

The eggs of these quadrupeds are formed in the same way as those
of birds, for they are hard-shelled and two-coloured, and they are
formed near the hypozoma as are those of birds, and in all other
respects resemble them both internally and externally, so that the
inquiry into their causes is the same for all. But whereas the eggs of
quadrupeds are hatched out by the mere heat of the weather owing to
their strength, those of birds are more exposed to destruction and
need the mother-bird. Nature seems to wish to implant in animals a
special sense of care for their young: in the inferior animals this
lasts only to the moment of giving birth to the incompletely developed
animal; in others it continues till they are perfect; in all that
are more intelligent, during the bringing up of the young also. In
those which have the greatest portion in intelligence we find
familiarity and love shown also towards the young when perfected, as
with men and some quadrupeds; with birds we find it till they have
produced and brought up their young, and therefore if the hens do
not incubate after laying they get into worse condition, as if
deprived of something natural to them.

The young is perfected within the egg more quickly in sunshiny
weather, the season aiding in the work, for concoction is a kind of
heat. For the earth aids in the concoction by its heat, and the
brooding hen does the same, for she applies the heat that is within
her. And it is in the hot season, as we should expect, that the eggs
are more apt to be spoilt and the so-called 'uria' or rotten eggs
are produced; for just as wines turn sour in the heats from the
sediment rising (for this is the cause of their being spoilt), so is
it with the yolk in eggs, for the sediment and yolk are the earthy
part in each case, wherefore the wine becomes turbid when the sediment
mixes with it, and the like applies to the eggs that are spoiling
because of the yolk. It is natural then that such should be the case
with the birds that lay many eggs, for it is not easy to give the
fitting amount of heat to all, but (while some have too little)
others have too much and this makes them turbid, as it were by
putrefaction. But this happens none the less with the birds of prey
though they lay few eggs, for often one of the two becomes rotten, and
the third practically always, for being of a hot nature they make
the moisture in the eggs to overboil so to say. For the nature of
the white is opposed to that of the yolk; the yolk congeals in
frosts but liquefies on heating, and therefore it liquefies on
concoction in the earth or by reason of incubation, and becoming
liquid serves as nutriment for the developing chick. If exposed to
heat and roasted it does not become hard, because though earthy in
nature it is only so in the same way as wax is; accordingly on heating
too much the eggs become watery and rotten, [if they be not from a
liquid residue]. The white on the contrary is not congealed by
frost but rather liquefies (the reason of which has been stated
before), but on exposure to heat becomes solid. Therefore being
concocted in the development of the chick it is thickened. For it is
from this that the young is formed (whereas the yolk turns to
nutriment) and it is from this that the parts derive their growth
as they are formed one after another. This is why the white and the
yolk are separated by membranes, as being different in nature. The
precise details of the relation of the parts to one another both at
the beginning of generation and as the animals are forming, and also
the details of the membranes and umbilical cords, must be learnt
from what has been written in the Enquiries; for the present
investigation it is sufficient to understand this much clearly,

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