In animals

where generation goes by heredity, wherever there is
duality of sex generation is due to copulation. In the group of
fishes, however, there are some that are neither male nor female,
and these, while they are identical generically with other fish,
differ from them specifically; but there are others that stand
altogether isolated and apart by themselves. Other fishes there are
that are always female and never male, and from them are conceived
what correspond to the wind-eggs in birds. Such eggs, by the way, in
birds are all unfruitful; but it is their nature to be independently
capable of generation up to the egg-stage, unless indeed there be some
other mode than the one familiar to us of intercourse with the male;
but concerning these topics we shall treat more precisely later on. In
the case of certain fishes, however, after they have spontaneously
generated eggs, these eggs develop into living animals;