We have seen that in the higher types of animals and plants the single individual is made up of not one but millions and millions of cells united together for the common purpose of the individual life, and that in such complicated individualities some cells perform one function while others perform others. A human individual from this point of view, therefore, is an organised community of cells all of which, however, sprang, in the first place, from one single cell. That original single cell is termed, in animal Embryology, the "fertilised ovum." It is popularly spoken of frequently as "the egg." All the other millions of cells are the direct descendants of this fertilised ovum, or egg, even though many of them eventually become extremely unlike the original cell. In single-celled animals the offspring of the original cell remain like the parent cell, but in the highly complicated creatures the offspring split up into a great many types of cells, owing to the very fact that all remain adherent together to form the mass of the body in order to carry out different functions. So we find cells of one type in glands, of another type in the brain, of another type in bones, of another type in blood, and so forth. Nevertheless all of them sprang from one original single cell. None of these specialised types of cells, however, are capable of performing any other function than their own. A bone-cell cannot receive an impression, nor originate an idea, any more than a brain-cell can secrete bile. Each kind of cell has its own appointed duty. The most important duty that can possibly be allotted to any cell is obviously that of reproducing the individual for the purpose of continuing the race or species. So we find in higher animals that this function, like others, is relegated to a special set of cells also derived from the original single cell, and which are called "the germ-cells." Leaving out of consideration the question of reproduction in lower types of animals we may consider the nature and origin of these cells in highest vertebrates, such as the mammals, including man. Germ-cells, which are derived from the tissues of a female animal, are termed "ova." Those which are derived from the tissues of a male animal are termed "sperms." Notice that it is not these germ-cells themselves to which the terms male and female, indicative of the two sexes, are applied, but only to the individuals. They are male and female; the germ-cells are of neither sex. True the germ-cells from the male, _i.e._ the sperms, differ in appearance when seen under the microscope from those of the female, but there is no reason to believe that there is any difference between them in their capacity, for example, of transmitting the characters of ancestors to succeeding generations.