During the period above referred to the chick
sleeps, wakes up, makes a move and looks up and Chirps; and the
heart and the navel together palpitate as though the creature were
respiring. So much as to generation from the egg in the case of birds.

Birds lay some eggs that are unfruitful, even eggs that are
the result of copulation, and no life comes from such eggs by
incubation; and this phenomenon is observed especially with pigeons.

Twin eggs have two yolks. In some twin eggs a thin partition
of white intervenes to prevent the yolks mixing with each other, but
some twin eggs are unprovided with such partition, and the yokes run
into one another. There are some hens that lay nothing but twin
eggs, and in their case the phenomenon regarding the yolks has been
observed. For instance, a hen has been known to lay eighteen eggs, and
to hatch twins out of them all, except those that were wind-eggs;
the rest were fertile (though, by the way, one of the twins is
always bigger than the other), but the eighteenth was abnormal or
monstrous.