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.