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History of Animals   

other annoyance or disturbance, then even though she had made up her
mind to lay she can keep the egg back in abeyance. A singular
phenomenon is observed in pigeons with regard to pairing: that is,
they kiss one another just when the male is on the point of mounting
the female, and without this preliminary the male would decline to
perform his function. With the older males the preliminary kiss is
only given to begin with, and subsequently sequently he mounts without
previously kissing; with younger males the preliminary is never
omitted. Another singularity in these birds is that the hens tread one
another when a cock is not forthcoming, after kissing one another just
as takes place in the normal pairing. Though they do not impregnate
one another they lay more eggs under these than under ordinary
circumstances; no chicks, however, result therefrom, but all such eggs
are wind-eggs.

Generation from the egg proceeds in an identical manner with all
birds, but the full periods from conception to birth differ, as has
been said. With the common hen after three days and three nights there
is the first indication of the embryo; with larger birds the
interval being longer, with smaller birds shorter. Meanwhile the
yolk comes into being, rising towards the sharp end, where the
primal element of the egg is situated, and where the egg gets hatched;
and the heart appears, like a speck of blood, in the white of the egg.
This point beats and moves as though endowed with life, and from it
two vein-ducts with blood in them trend in a convoluted course (as the
egg substance goes on growing, towards each of the two circumjacent
integuments); and a membrane carrying bloody fibres now envelops the
yolk, leading off from the vein-ducts. A little afterwards the body is
differentiated, at first very small and white. The head is clearly
distinguished, and in it the eyes, swollen out to a great extent. This
condition of the eyes lat on for a good while, as it is only by
degrees that they diminish in size and collapse. At the outset the
under portion of the body appears insignificant in comparison with the
upper portion. Of the two ducts that lead from the heart, the one
proceeds towards the circumjacent integument, and the other, like a
navel-string, towards the yolk. The life-element of the chick is in
the white of the egg, and the nutriment comes through the navel-string
out of the yolk.
When the egg is now ten days old the chick and all its parts are
distinctly visible. The head is still larger than the rest of its
body, and the eyes larger than the head, but still devoid of vision.
The eyes, if removed about this time, are found to be larger than
beans, and black; if the cuticle be peeled off them there is a white
and cold liquid inside, quite glittering in the sunlight, but there is
no hard substance whatsoever. Such is the condition of the head and
eyes. At this time also the larger internal organs are visible, as
also the stomach and the arrangement of the viscera; and veins that
seem to proceed from the heart are now close to the navel. From the
navel there stretch a pair of veins; one towards the membrane that
envelops the yolk (and, by the way, the yolk is now liquid, or more so
than is normal), and the other towards that membrane which envelops
collectively the membrane wherein the chick lies, the membrane of
the yolk, and the intervening liquid. (For, as the chick grows, little
by little one part of the yolk goes upward, and another part downward,
and the white liquid is between them; and the white of the egg is
underneath the lower part of the yolk, as it was at the outset.) On
the tenth day the white is at the extreme outer surface, reduced in
amount, glutinous, firm in substance, and sallow in colour.
The disposition of the several constituent parts is as
follows. First and outermost comes the membrane of the egg, not that
of the shell, but underneath it. Inside this membrane is a white
liquid; then comes the chick, and a membrane round about it,
separating it off so as to keep the chick free from the liquid; next

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