e of birds; and this because, to the minute
essential portion of the egg, or germ, from which the young bird grows,
there is added a large store of food-material--the yolk and white of the
egg--destined to nourish the growing embryo while the whole is enclosed
within a hard shell.

The relative sizes of eggs depend entirely on the amount of the
food-yolk thus enclosed with the germ; while the form and texture of the
outer envelope are determined by the nature of the environment to which
the egg is exposed. Where the food material is infinitesimal in quantity
the egg is either not extruded--the embryo being nourished by the
maternal tissues,--or it passes out of the parental body and gives rise
at once to a free-living organism or "larva" (see LARVAL FORMS), as in
the case of many lowly freshwater and marine animals. In such cases no
"egg" in the usual sense of the term is produced.

The number of eggs periodically produced by any given individual depends
on the risks of destruction to which they, and the young to which they
give rise, are exposed: not more than a single egg being annually laid
by some species, while with others the number may amount to millions.

_Birds' Eggs._--The egg of the bird affords, for general purposes, the
readiest example of the modifications imposed on eggs by the external
environment. Since it must be incubated by the warmth of the parent's
body, the outer envelope has taken the form of a hard shell for the
protection of the growing chick from pressure, while the dyes which
commonly colour the surface of this shell serve as a screen to hide it
from egg-eating animals.

Carbonate of lime forms the principal constituent of this shell; but in
addition phosphate of lime and magnesia are also present. In section,
this shell will be found to be made up of three more or less distinct
crystalline layers, traversed by vertical canals, whereby the shell is
made porous so as to admit air to the developing chick.

The outermost, or third, layer of this shell often takes the form of a
glaze, as of porcelain, as for example in the burnished egg of the
ostrich: or it may assume the character of a thick, chalky layer as in
some cuckoos (_Guira_, _Crotophaga ani_), cormorants, grebes and
flamingoes: while in some birds as in the auks, gulls and tinamous, this
outer layer is wanting; yet the tinamous have the most highly glazed
eggs of all birds, the second layer of the shell developing a surface
even