ntless transparent spheres, each of
the size of a small pin's head. These are the eggs of our common Natica
or Sea-Snail. Any one who remembers the outline of this shell will
easily understand the process by which its eggs are left lying on the
beach in the form I have described. They are laid in the shape of a
broad, short ribbon, pressed between the mantle and the shell, and,
passing out, cover the outside of the shell, over which they are rolled
up, with a kind of glutinous envelope,--for the eggs are held together
by a soft glutinous substance. Thus surrounded, the shell, by its
natural movements along the beach, soon collects the sand upon it, the
particles of which in contact with the glutinous substance of the eggs
quickly forms a cement that binds the whole together in a kind of paste.
When consolidated, it drops off from the shell, having taken the mould
of its form, as it were, and retaining the curve which distinguishes the
outline of the Natica. Although these saucers look perfectly round, it
will be found that the edges are not soldered together, but are simply
lapped one over the other. Every one of the thousand little spheres
crowded into such a circle of sand contains an egg. If we follow the
development of these eggs, we shall presently find that each one divides
into two halves, these again dividing to make four portions, then the
four breaking up into eight, and so on, till we may have the yolks
divided into no less than sixteen distinct parts. Thus far this process
of segmentation is similar to that of the egg in other animals; but, as
we shall see hereafter, it seems usually to result only in a change in
the quality of its substance, for the portions coalesce again to form
one mass, from which a new individual is finally sketched out, at first
as a simple embryo, and gradually undergoing all the changes peculiar to
its kind, till a new-born animal escapes from the egg. But in the case
of the Natica this regular segmentation changes its character, and at a
certain period, in a more or less advanced stage of the segmentation,
according to the species, each portion of the yolk assumes an
individuality of its own, and, instead of uniting again with the rest,
begins to subdivide for itself. In our _Natica heros_, for instance, the
common large gray Sea-Snail of our coast, this change takes place when
the yolk has subdivided into eight parts. At that time each portion
begins a life of its own, not reuniting