At a certain stage in the life history of the animal individual and after undergoing certain changes which need not be considered here, these germ-cells, both sperms and ova, have reached such a stage of maturity as to be capable of carrying on their sole function, namely, that of reproducing the species. The actual age in human beings, for example, at which this maturity is reached varies very much in different races, and in different individuals of the same race. When reproduction is about to occur a union must take place between a germ-cell from a male body with a germ-cell from a female body; that is to say, a union must take place between a sperm and an ovum. This union takes place within the body of the female individual and results in the fusion of the two cells into one single cell, which is now termed a fertilised ovum. This fertilised ovum, in virtue of this process of union, is now able under suitable conditions of nutrition and shelter, such as it obtains within the female organs of reproduction, to divide and redivide again and again, thus building up a new mass of cells as the result of its division. The millions of cells so produced include, as we have already seen, cells which have all the various functions which are necessary for the continuation of the life of a human individual; that is to say, that as the result of this division of the fertilised ovum there are produced first of all germ-cells to secure the still further continuance of the race, and then multitudes of all the other kinds of cells which gradually assume the shape of an embryo or young individual, and ultimately grow into a human being.