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egg is not to be found in any of the famous collections on which oologists have spent fortunes, and for the acquisition of which museums have set apart large sums. Though no one ever lavished upon it such amounts as have been paid for an egg of the Great Auk or the extinct Dodo, it is practically as seldom seen, and as little known, as any of the rare eggs which collectors covet so ardently and prize so highly.

We usually think of eggs as consisting of a yolk surrounded by albumen, and enclosed in a hard shell made of lime or some form of calcium. The eggs of birds and fowls are our types, but the egg of the Earthworm has no chalk-like shell. Most eggs, together with their shells, are formed within the body of the egg-bearing animal, but this egg differs from the majority in this respect. It is true that the egg itself is formed as usual in the ovary, and passed through a tube known as the oviduct, but the shell or case is fabricated by the animal externally, and is slipped over the egg as it passes out of the oviduct and is about to be deposited. Eggs are very commonly laid in a nest, more or less elaborately constructed, and it is a rare thing for only one egg to be laid during the season by each individual. It has been correctly surmised that the number of eggs laid by a bird or other animal bears a close relation to the exigencies and dangers which the young will be likely to encounter. Hence a Pigeon lays only a single pair of eggs for each brood, while the Thrush deposits some half-dozen in its nest; and Partridges, Pheasants, Tits, and other birds lay from a dozen to a score. Then we find that Herrings and other fish lay enormous quantities of eggs as compared with many fresh-water species.