yolk, enclosed within a shell, and
extruded to undergo development apart from the maternal tissues. In the
case of the echidna the eggs, two in number, are about as large as those
of a sparrow, similar in shape, and have a white, parchment-like shell.
After expulsion they are transferred by the beak of the mother to a
pouch resembling that of the marsupial kangaroos, and there they undergo
development. The _Ornithorhynchus_, on the other hand, lays from two to
four eggs, which in size and general appearance resemble those of the
echidna. They are, however, deposited in a loosely constructed nest at
the end of a long burrow and there brooded. In Marsupials, the eggs are
smaller than those of _Echidna_ and _Ornithorhynchus_, and they contain
a larger proportion of yolk than occurs in higher mammals.

_Eggs of Reptiles._--The eggs of reptiles are invariably provided with a
large amount of food yolk and enclosed with a firm test or shell, which
though generally parchment-like in texture may be calcareous as in
birds, as, for example, in many of the tortoises and turtles and in the
crocodiles.

Among reptiles the egg is always white or yellowish, while the number
laid often far exceeds that in the case of birds. The tuatara of New
Zealand, however, lays but ten--white hard-shelled, long and oval--at
intervals between November and January. The long intervals between the
appearance of the successive eggs is a characteristic feature of the
reptiles, but is met with among the birds only in the megapodes, which,
like the reptiles, do not "brood" their eggs.

Among the Chelonia the number of eggs varies from two to four in some of
the tortoises, to 200 in some of the turtles: while in the crocodiles
between 20 and 30 are produced, hard-shelled and white.

The eggs of the lizards are always white or yellowish, and generally
soft-shelled; but the geckos and the green lizard lay hard-shelled eggs.
Many of the soft-shelled eggs are remarkable for the fact that they
increase in size after extrusion, owing to the stretching of the
membranous shell by the growing embryo. In the matter of number lizards
are less prolific than many of the Chelonia, a dozen eggs being the
general number, though as many as thirty may be produced at a time, as
in the case of the common chameleon.

While as a general rule the eggs of lizards are laid in burrows or
buried, some are retained within the body of the parent until the young
are ready to emer