death of the cells. If they be so interfered with as to be destroyed it is hardly possible for the cell itself to continue to exist. One of the most interesting examples of this resistance of germ-cells to their environment is in connection with some human diseases which have existed from time immemorial, diseases the descriptions of which are to be found given quite accurately in the most ancient documents, but in spite of the fact that human germ-cells have been subject to the hostile surroundings which such human diseases involve they themselves have not changed to any great extent. That is to say they still produce a type of embryo and offspring practically identical with that that always was produced.

The same truth applies to the cells which make up the body of the embryo and the individual, as well as to the germ-cells. The body-cells, those which make up bone, and muscle, and gland, and so forth, are constantly exposed to all sorts of influences which must tend to damage them so far as it is possible for the cells to be damaged and still live. These body-cells are sometimes starved, sometimes poisoned with alcohol and drugs, frozen by extremes of temperature, over-worked by too much physical strain, and so on, and if it were possible for such external influences to change the type of cells of their offspring we should expect to see it here. But it does not occur. The internal hereditary tendencies of these cells are so strong, and so intimately bound up with the life of the cells themselves, that when they divide and produce others these others are precisely similar to the parent cells, in spite of all the unfavourable environment in which they have been. Slight variations do, of course, occur, but these are chiefly of a germinal or spontaneous nature, and not due to the environment.

This thought gives us some vague and imperfect idea of how immensely complex the constitution of germ-plasm must be. This germ-plasm is very often subjected to all sorts of unfavourable conditions, especially those of alcohol and toxins, and such conditions have been acting upon it more or less for an immense number of generations, and yet the resistance to modification at the hands of these internal factors is so great that all the processes which follow upon the fertilisation of the ovum, all the thousand complications which thereafter ensue in the building up of the young embryo are hardly ever interfered with. When they are markedly interfered with such interference generally involves the death of the embryo.

The conclusion arrived at on this subject by Dr. Archdall Reid, after a very careful and extensive inquiry into all the evidence from many points of view, is stated by him as follows: “Though variations may result from the direct action of the environment, such variations are, in effect, always injuries, and are of rare occurrence in individuals who survive and have offspring. Adaptation (i.e. evolution) depends almost exclusively on spontaneous variations. These do not imply damage to the germ-plasm, but are products of its vital activity. Occurring in vast abundance all round the specific and parental means, they supply the sole material for Natural Selection.”

“We conceive the germ-plasm, then, as living and active, closely adjusted to its environment, growing, dividing, varying, capable of being destroyed and injured, but resisting death and injury, and within limits capable of repairing damage and returning to its original state—as behaving exactly as a living individual does.”

CHAPTER IV

THE MAKING OF A MAN

Having in this brief preliminary consideration of the fundamental facts upon which the science of Embryology is based cleared the ground as far as possible, we may now summarise, in a few simple statements, the point at which we have arrived in order that we may proceed at once to the more detailed study of the actual development of the embryo itself.

We are in search of as clear a statement as possible of the origin of the many and varied characteristics which go to the formation of a human embryo, and hence to the making of an individual. The variation in these many characteristics accounts for the differences in individualities. No two individuals are exactly similar whatever be the standard by which we estimate them. This is true morally, ethically, and physically. In each of these spheres there are to be found good, bad, and indifferent individuals, but whichever they are it is quite obvious that the result has been brought about by the influence of all the factors of heredity and environment acting upon the capacities which were originally implanted in the germ-plasm. An individual is the resultant of the play upon one man's-worth of human material of all forces which have acted, or are acting, upon that kind and amount of material. Even though two children of the same parents be brought up under what are to all appearances identical circumstances, they differ from the very beginning from each other and their parents. This is true even of physical characteristics, and even more markedly in mental features. The fact is—and it is one which is not sufficiently recognised—that the formation of an individual from an embryo, the making of a man, is a biological problem fundamentally.

The following are the principal facts which we have at this stage to bear in mind.

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