( Originally Published 1923 )
Sexual Reproduction.—Sexual reproduction occurs among plants as well as among animals and among the lowest forms of life as well as the highest. Under certain conditions two cells, that is, two independent individuals, meet, unite into one, and then this new cell, consisting of protoplasm derived from two preexisting individuals, becomes the new generation. The fusing of two cells into one is the basic fact of sex.
When we examine plants and animals -consisting of many cells, we find that certain special cells are alone capable of uniting with one another; and that two kinds of uniting cells are formed, the union always taking place between two cells of different kinds.
A cell which is capable of uniting or conjugating with another is called a gamete. Among those plants and animals that have two different kinds of gametes one kind is called the, male, the other the female. The male gametes or sperm cells and the female gametes or egg cells are produced in special organs ; and among all of the higher classes of animals and among many species of plants, these are borne by two different kinds of individuals, which we call accordingly males and females.
Where the two gametes are exactly alike, as they are among the lowest plants and animals, we still speak of sexual reproduction, although the terms male and female have no application. But when the two gametes differ from each other, maleness and femaleness become characteristic of every phase of the individual's life.
The most striking difference between the female gamete and the male is one of size. The larger one is always the female. Related to the larger size is the greater quantity of food material and the smaller amount of movement to be seen in the female gamete. It is the male sperm that moves, whereas the female gamete or egg cell remains at rest. Associated with the fact of movement in the sperm cell is the further fact of special sensitiveness or irritability, which determines the direction in which the sperm cell will swim. The egg cells of plants and animals in which fertilization takes place in the water give off a substance that attracts the sperm cells.
The two kinds of gametes may then be compared as follows :
Large Small Round Head with swimming apparatus Resting Motile
The essential facts of sexual reproduction lie in: (1) the forming of gametes, (2) their liberation, (3) their meeting, and (4) their uniting. All the rest of life which is not directly concerned with supplying the individual with the essentials of life is directly or indirectly related to sex.