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.