pinging energies, or stimuli, in a manner which is usually, but not invariably, adaptive or useful.
Both the egg and the sperm are living cells with typical cell structures and functions, but with none of the parts of the mature organism into which they may develop. But-although they do not contain any of the differentiated structures and functions of the developed organism, they differ from other cells in that they are capable under suitable conditions of producing these structures and functions by the process of development or differentiation, in the course of which the general structures and functions of the germ cells are converted into the specific structures and functions of the mature animal or plant.
In both plants and animals the sex cells are fundamentally alike, though they differ greatly in appearances. The female sex cells of flowering plants are called ovules, the male cells pollen. The corresponding cells of animals are known as ova and spermatozoa. Collectively all kinds of sex cells are called gametes, and the individual formed by the union of a male and female gamete is known as a zygote, while the cell formed by the union of egg and sperm is frequently called the oosperm.
The egg cell of animals is usually spherical in form and contains more or less food substance in the form of yolk; it varies greatly in size, depending chiefly upon the quantity of yolk, from the great egg of a bird, in which the yolk, or egg proper, may be hundreds of millimeters in diameter, to the miscroscopic eggs of oysters and worms, which may be no more than a few thousandths of a millimeter in diameter. The human ovum is microscopic in size (about 0.2 mm. in diameter) but it is not smaller than is found in many other animals. It has all the characteristic parts of any egg cell, and can not be distinguished microscopically from the eggs of several other mammals, yet there is no doubt that the ova of each species differ from those of every other species, and later we shall see reasons for concluding that the ova produced by each individual are different from those produced by any other individual.
The sperm, or male gamete, is among the smallest of all cells and is usually many thousands of times smaller than the egg. In most animals, and in all vertebrates, it is an elongated, thread-like cell with an enlarged head which contains the nucleus, a smaller middle-piece, and a . very long and slender tail or flagellum, by the lashing of which the spermatozoon swims forward in the jerking fashion characteristic of many monads or flagellated protozoa. In different species of animals the spermatozoa often differ in size and appearance, and there is every reason to believe that the spermatozoa of each species are peculiar in certain respects even though we may not be able to distinguish any structural differences under the microscope. The human spermatozoa closely resemble those of other primates but are still slightly different, and the conclusion is inevitable, as we shall see later, that the sperma-
