But in birds, he says, since copulation
takes place quickly, both the eggs and their colour always cross one
another. But if it is the fact, as it manifestly is, that several
young are produced from one emission of semen and a single act of
intercourse, it is better not to desert the short road to go a long
way about, for in such cases it is absolutely necessary that this
should occur when the semen is not separated but all enters the female
at once.
If, then, we must attribute the cause to the semen of the male, this
will be the way we shall have to state it, but we must rather by all
means suppose that the cause lies in the material contributed by the
female and in the embryo as it is forming. Hence also such
monstrosities appear very rarely in animals producing only one young
one, more frequently in those producing many, most of all in birds and
among birds in the common fowl. For this bird produces many young, not
only because it lays often like the pigeon family, but also because it
has many embryos at once and copulates all the year round. Therefore
it produces many double eggs, for the embryos grow together because
they are near one another, as often happens with many fruits. In
such double eggs, when the yolks are separated by the membrane, two
separate chickens are produced with nothing abnormal about them;
when the yolks are continuous, with no division between them, the
chickens produced are monstrous, having one body and head but four
legs and four wings; this is because the upper parts are formed
earlier from the white, their nourishment being drawn from the yolk,
whereas the lower part comes into being later and its nourishment is
one and indivisible.
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