Embryology: The Beginnings of Life

by Gerald R. Leighton

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 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.

We therefore see that the embryo, or the individual, is formed from one, and one only, of the first products of the division of the fertilised germ-cell, the rest of these products forming the other body tissues. This idea of the continuity of the germ-plasm is the greatest contribution of modern embryological research. It is quite fundamental, and no clear understanding of what is involved in the making of a man is possible without it. It teaches us that the line of ancestry and heredity is from one generation of germ-cells to another, directly, and never through the individual from the embryo, which, indeed, is a mere side product in the continued chain of events. The individual is practically the trustee of the germ-cells, but not their maker. No embryo, and no individual, ever made germ-cells; the latter existed first. The object of the embryo is obviously to protect and nourish the germ-cells which have been placed within it, so that they may be available in due time for the production of further germ-cells, and so for the continuity of the race. Hence it is the all-pervading truth of natural selection that the interest and survival of the individual is almost of no account; that of the species or the race being the paramount consideration.

Once these facts be grasped there is no longer any difficulty in understanding why the process of reproduction in any given species always results in the formation of embryos which resemble each other in all the main characters of their species. It could not be otherwise, because they come from portions of identically similar material, a common germ-plasm. In other words, the individual inherits nothing from its parents. He merely receives in his turn the material inheritance in the germ-plasm which was there a generation before him.

In so far as there has been no germinal variation he and they will be similar. Hence the common observation that the child resembles the parent. True, so he does; but not because he gets his characters from them, but simply because he and they obtain their characteristics from a common source. To many this thought will be, perhaps, a new one. It is one of the most interpreting ideas which science has given us, and in its absence no real grasp of the origin of the physical, mental, and other characters if there be any, of the embryo can be understood. The present writer has elsewhere summarised this thought as follows:—