No one who has often walked across a sand-beach in summer can have failed to remark what the children call “sand saucers.”  The name is not a bad one, with the exception that the saucer lacks a bottom; but the form of these circular bands of sand is certainly very like a saucer with the bottom knocked out.  Hold one of them against the light and you will see that it is composed of countless 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 with its seven twin portions; so that in the end, instead of a single embryo growing out of this yolk, we have eight embryos arising from a single yolk, each one of which undergoes