All cartilaginous fishes have at one and the same time eggs
above close to the midriff (some larger, some smaller), in
considerable numbers, and also embryos lower down. And this
circumstance leads many to suppose that fishes of this species pair
and bear young every month, inasmuch as they do not produce all
their young at once, but now and again and over a lengthened period.
But such eggs as have come down below within the womb are
simultaneously ripened and completed in growth.

Dog-fish in general can extrude and take in again their young,
as can also the angel-fish and the electric ray-and, by the way, a
large electric ray has been seen with about eighty embryos inside
it-but the spiny dogfish is an exception to the rule, being
prevented by the spine of the young fish from so doing. Of the flat
cartilaginous fish, the trygon and the ray cannot extrude and take
in again in consequence of the roughness of the tails of the young.
The batrachus or fishing-frog also is unable to take in its young
owing to the size of the head and the prickles; and, by the way, as
was previously remarked, it is the only one of these fishes that is
not viviparous.

So much for the varieties of the cartilaginous species and for
their modes of generation from the egg.

The so-called smooth shark has its eggs in betwixt the wombs
like the dog-fish; these eggs shift into each of the two horns of
the womb and descend, and the young develop with the navel-string
attached to the womb, so that, as the egg-substance gets used up,
the embryo is sustained to all appearance just as in the case of
quadrupeds. The navel-string is long and adheres to the under part
of the womb (each navel-string being attached as it were by a sucker),
and also to the centre of the embryo in the place where the liver is
situated. If the embryo be cut open, even though it has the
egg-substance no longer, the food inside is egg-like in appearance.
Each embryo, as in the case of quadrupeds, is provided with a
chorion and separate membranes. When young the embryo has its head
upwards, but downwards when it gets strong and is completed in form.
Males are generated on the left-hand side of the womb, and females
on the right-hand side, and males and females on the same side
together. If the embryo be cut open, then, as with quadrupeds, such
internal organs as it is furnished with, as for instance the liver,
are found to be large and supplied with blood.

As regards the shape of the womb, the reader is referred to my
treatise on Anatomy. The womb, however, is diverse in diverse
fishes, as for instance in the sharks as compared one with another
or as compared with the skate. That is to say, in some sharks the eggs
adhere in the middle of the womb round about the backbone, as has been
stated, and this is the case with the dog-fish; as the eggs grow
they shift their place; and since the womb is bifurcate and adheres to
the midriff, as in the rest of similar creatures, the eggs pass into
one or other of the two compartments. This womb and the womb of the
other sharks exhibit, as you go a little way off from the midriff,
something resembling white breasts, which never make their
appearance unless there be conception.

Dog-fish and skate have a kind of egg-shell, in the which is
found an egg-like liquid. The shape of the egg-shell resembles the
tongue of a bagpipe, and hair-like ducts are attached to the shell.
With the dog-fish which is called by some the 'dappled shark', the
young are born when the shell-formation breaks in pieces and falls
out; with the ray, after it has laid the egg the shell-formation
breaks up and the young move out. The spiny dog-fish has its close
to the midriff above the breast like formations; when the egg
descends, as soon as it gets detached the young is born. The mode of
generation is the same in the case of the fox-shark.