GANGLION. — A swelling or knot from which nerves are given off as from a centre.

GANOID FISHES. — Fishes covered with peculiar enamelled bony scales. Most of them are extinct.

GERMINAL VESICLE. — A minute vesicle in the eggs of animals, from which the development of the embryo proceeds.

GLACIAL PERIOD. — A period of great cold and of enormous extension of ice upon the surface of the earth. It is believed that glacial periods have occurred repeatedly during the geological history of the earth, but the term is generally applied to the close of the Tertiary epoch, when nearly the whole of Europe was subjected to an arctic climate.

GLAND. — An organ which secretes or separates some peculiar