The navel-string is attached a little way below the aperture
of the belly. When the creatures are young the navel-string is long,
but as they grow it diminishes in size; at length it gets small and
becomes incorporated, as was described in the case of birds. The
embryo and the egg are enveloped by a common membrane, and just
under this is another membrane that envelops the embryo by itself; and
in between the two membranes is a liquid. The food inside the
stomach of the little fishes resembles that inside the stomach of
young chicks, and is partly white and partly yellow.

Development from the egg in fishes differs from that in birds in
this respect, that it does not exhibit that one of the two
navel-strings that leads off to the membrane that lies close under the
shell, while it does exhibit that one of the two that in the case of
birds leads off to the yolk. In a general way the rest of the
development from the egg onwards is identical in birds and fishes.
That is to say, development takes place at the upper part of the
egg, and the veins extend in like manner, at first from the heart; and
at first the head, the eyes, and the upper parts are largest; and as
the creature grows the egg-substance decreases and eventually
disappears, and becomes absorbed within the embryo, just as takes
place with the yolk in birds.

So much for the conception and generation of birds.

It has been previously stated that fishes are not all oviparous.
Fishes of the cartilaginous genus are viviparous; the rest are
oviparous. And cartilaginous fishes are first oviparous internally and
subsequently viviparous; they rear the embryos internally, the
batrachus or fishing-frog being an exception.

Fishes also, as was above stated, are provided with wombs, and
wombs of diverse kinds. The oviparous genera have wombs bifurcate in
shape and low down in position; the cartilaginous genus have wombs
shaped like those of O birds. The womb, however, in the
cartilaginous fishes differs in this respect from the womb of birds,
that with some cartilaginous fishes the eggs do not settle close to
the diaphragm but middle-ways along the backbone, and as they grow
they shift their position.

The egg with all fishes is not of two colours within but is of
even hue; and the colour is nearer to white than to yellow, and that
both when the young is inside it and previously as well.

Peafowl live for about twenty-five years, breed about the third
year, and at the same time take on their spangled plumage. They
hatch their eggs within thirty days or rather more. The peahen lays
but once a year, and lays twelve eggs, or may be a slightly lesser
number: she does not lay all the eggs there and then one after the
other, but at intervals of two or three days. Such as lay for the
first time lay about eight eggs. The peahen lays wind-eggs. They
pair in the spring; and laying begins immediately after pairing. The
bird moults when the earliest trees are shedding their leaves, and
recovers its plumage when the same trees are recovering their foliage.
People that rear peafowl put the eggs under the barn-door hen, owing
to the fact that when the peahen is brooding over them the peacock
attacks her and tries to trample on them; owing to this circumstance
some birds of wild varieties run away from the males and lay their
eggs and brood in solitude. Only two eggs are put under a barn-door
hen, for she could not brood over and hatch a large number. They
take every precaution, by supplying her with food, to prevent her
going off the eggs and discontinuing the brooding.

With male birds about pairing time the testicles are obviously
larger than at other times, and this is conspicuously the case with
the more salacious birds, such as the barn-door cock and the cock
partridge; the peculiarity is less conspicuous in such birds as are
intermittent in regard to pairing.

With most birds, as has been said of the pigeon, the hatching is
carried on by the male and the female in turns: with some birds,
however, the male only sits long enough to allow the female to provide
herself with food. In the goose tribe the female alone incubates,
and after once sitting on the eggs she continues brooding until they
are hatched.

The nests of all marsh-birds are built in districts fenny and well
supplied with grass; consequently, the mother-bird while sitting quiet
on her eggs can provide herself with food without having to submit
to absolute fasting.

With the crow also the female alone broods, and broods
throughout the whole period; the male bird supports the female,
bringing her food and feeding her. The female of the ring-dove
begins to brood in the afternoon and broods through the entire night
until breakfast-time of the following day; the male broods during
the rest of the time. Partridges build a nest in two compartments; the
male broods on the one and the female on the other. After hatching,
each of the parent birds rears its brood. But the male, when he
first takes his young out of the nest, treads them.