When the result of conception is

perfected, with some animals a living creature is brought forth,
with others an egg is brought to light, with others a grub. Of the
eggs, some have egg-shells and are of two different colours within,
such as birds' eggs; others are soft-skinned and of uniform colour, as
the eggs of animals of the shark kind. Of the grubs, some are from the
first capable of movement, others are motionless. However, with regard
to these phenomena we shall speak precisely hereafter when we come
to treat of Generation.

Furthermore, some animals have feet and some are destitute
thereof. Of such as have feet some animals have two, as is the case
with men and birds, and with men and birds only; some have four, as
the lizard and the dog; some have more, as the centipede and the
bee; but allsoever that have feet have an even number of them.