and enclosing a central sphere of yolk, the upper pole of which
is black. The spawn of the common frog differs from that of the toad in
that the eggs all adhere to form a huge jelly-like mass. But in many
species the number of eggs produced are few; and these may be
sufficiently stored with food-yolk to allow of the tadpole stage being
passed before hatching, as in frogs of the genus _Hylodes_. In many
cases the eggs are deposited out of the water and often in quite
remarkable ways.

_Eggs of Fishes._--The eggs of fishes present an extremely wide range of
form, and a no less extensive range in the matter of number. Both among
the cartilaginous and bony fishes viviparity occurs. Most of the sharks
and rays are viviparous, but in the oviparous species the eggs present
some interesting and peculiar forms. Large in size, the outer coat or
"shell" is in all cases horn-like and flexible, but differs greatly in
shape. Thus in the egg of the larger spotted dog-fish it is oblong in
shape, flattened from side to side, and has the angles produced into
long, slender tendrils. As the egg is laid the lower tendrils project
from the vent, and the mother rubs herself against some fixed body. The
tendrils soon catch fast in some slight projection, when the egg is
dragged forth there to remain till hatching takes place. A couple of
narrow slits at each corner of the upper end serve to admit fresh water
to the imprisoned embryo during the later stages of development; when
development is complete escape is made through the end of the shell. In
the rays or "skates," long spines take the place of tendrils, the egg
simply resting at the bottom of the sea. The empty egg-cases of the rays
are often found on the seashore, and are known as "Mermaids' purses."
The egg of the Port Jackson shark (_Cestracion_) is of enormous size,
pear-shaped, and provided with a spiral flange extending along the whole
length of the capsule. In the _Chimaera_ the egg is long, more or less
spindle-shaped, and produced on each side into a broad flange having a
fringed edge, so that the whole bears a close resemblance to a long
leaf, broad and notched at one end, pointed at the other. This likeness
to the seaweed among which it rests is doubtless a protective device,
akin to that of protectively coloured birds' eggs.

Among the bony fishes the eggs generally take the form of small spheres,
enclosed within a tough membrane or capsule. But they present many
important