All living creatures are made of cells, the physical basis of which is protoplasm. The simplest creatures consist of one such mass of protoplasm; higher organisms consist of more than one, and often of millions, in which case they adhere together. Cells multiply by dividing into two, the protoplasm of the mother-cell giving rise to that of the daughter-cells. A human embryo, therefore, which is going to give rise to an adult individual is a community consisting of an enormous number of cells, the whole of which have descended from one common ancestor, a single cell known as a fertilised ovum. True, these descendants break up into many types of cells in order that different functions may be performed by special tissues, but none of these special cells can do everything that is necessary for the life of the whole individual; they can only play their own special small part. They can do nothing towards continuing the species of the individual. This duty, like others, is imposed upon one particular group and kind of cells, namely, the germ-cells, which do nothing else in the animal economy but furnish the means for the continuity of the race. Although they lie within the tissues of the embryo, and afterwards of the adult, they take no part in the life of that embryo or adult. They undergo certain changes in themselves which are to fit them for their ultimate destiny and function, but they contribute nothing to the output of energy on the part of the individual. When these are derived from a female they are termed "ova"; when from a male they are termed "sperms." They themselves are neither male nor female, they are merely protected and nourished by the general mass of cells which constitutes the male and female individual. When a male germ-cell or sperm unites with a female germ-cell or ovum, within the female body, fertilisation of the ovum takes place, and this gives rise to the fertilised germ-cell from which is to arise first the germ-cells or direct descendants of itself, and secondly the embryo in which these germ-cells will come to lie. This happens by the repeated and continued division of the fertilised germ-cell, a division which constitutes growth, and which under suitable conditions of nourishment and protection and exercise will ultimately produce a human being. The great mass of the cells of this individual, the body or somatic cells, take no part whatsoever in giving rise to the germ-cells of the next generation. These are produced from the pre-existing germ-cells, and from no other source, and it is for this reason alone that the phenomena of heredity are possible and that one generation is directly continuous with its predecessors. In fact heredity may be defined as the relationship which exists between successive generations.