ich the young frog grows and the yolk on
which the growing animal feeds. By the untrained eye nothing more can be
seen in the frog's egg than a small black ball enclosed within a clear
jelly-like substance. At the time the egg is laid this outer jelly is
hardly noticeable, but it soon swells up, and thus forms a soft, elastic
covering to the growing frog, effectually protecting it from injury.
This black ball, by the way, answers to the yellow yolk of the hen's
egg: it differs from the yellow yolk in that it is colourless
internally, and black externally. The black outside coat apparently
serves to attract the heat of the sun, and thereby to bring about the
hatching process, which the hen does by the warmth of her own body.

[Illustration: Fig. 1.--A to G: Stages in the growth of the Tadpole,
greatly magnified.]

These eggs are furthermore remarkable in that they are laid, not one by
one, as a hen lays, but in thousands, and in water, forming an enormous
speckled mass. Take a portion of such a mass and watch it. Day by day
you will see the black spot gradually assume a distinct shape (fig. 1,
A): a little later a head and tail can be made out.

[Illustration: Fig. 2.--Mouth of Tadpole, greatly magnified.]

In a few hours more little black buds grow out on each side of the head,
and these soon become branched. They are the future gills. At this time
you will notice slight movements within this glassy cradle; and soon
after this the young frog, or tadpole, as we must call him now, escapes;
that is to say, as soon as he leaves his cradle he becomes a tadpole. At
first he does nothing but hang on to bits of weed, or the broken remains
of the covering of the egg, by a sticky substance formed by a special
pair of suckers placed just behind the mouth, as shown in our
illustration (fig. 2).

[Illustration: Fig. 3.--Gill of Tadpole, greatly magnified.]

Soon signs of life become apparent in the shape of a slow curving of the
body from side to side. In a very short time, however, these movements
increase so rapidly that the tail can hardly be seen, and at last, in
one of these violent wriggles he finds himself actually swimming! During
all this time he has swallowed no food, but has lived on the remains of
the egg within him; swallowing, indeed, has been out of the question,
for as yet his mouth is sealed! But now, at last, the little jaws are
unlocked, and he begins to eat ravenously, at first delicate green weed,
and late