236 AMPHIBIANS

some seventy species (B. vulgaris and B. calamtta,
the natterjack) are natives of Britain. Like all the
other toads, they secrete a certain mildly-poisonous
substance in their skin-glands, although there is one
really venomous toad, which is found in Argentina.
The tree-frogs (Hyde) have teeth in their upper
jaw, but are otherwise like the toads in structure.

In the frogs (Ranzd@) the sacral vertebra is not
dilated, and there are generally teeth in the upper
jaw. As there are two toads in this country, so
are there two frogs: one (Rana temporaria) said to
be native, the other (2. esculenta) found mostly in
Norfolk, said to have been acclimatised from the
Continent, and distinguishable from the common
species by the absence of the black patch on the
side of the head. The frog undergoes many changes
before he reaches maturity. The chief stages, as

_far as externals are concerned, are shown in our
illustration. At 1 we have the egg as just laid ;
at 2 we have it when the water has made its way
in through the enclosing membrane and distended
it, so as to leave the egg plenty of room; at 3 we
have the young, after the egg has been laid about ten
days, just before he is hatched; at 4 we have him
about four days afterwards, as he makes his first
appearance in the world, consisting apparently of
only tail and head—that is, a tailed-poll, or tadpole,
with gills on his neck; at 5 he has got a mouth,
for up to now he has lived without one; at 6 he
has got his front legs and is losing his gills; at 7
he has got his hind legs, he is working his lungs,
and beginning to feed on his tail. As the supply it