Embryology: The Beginnings of Life

by Gerald R. Leighton

one of which, and one only, goes on dividing to form the body-cells, and so produces the new individual, which as it grows includes in itself those cells (germ-cells) previously formed.

The rest are germ-cells, which subsequently form the eggs and sperms of the new individual, i.e. they are the germ-cells of the next generation. They cannot develop independently, but when they unite with the egg or sperm of another individual, a new fertilised ovum is formed and the cycle begins again.

Diagram to show the origin of germ-cells and the embryo.

This is a startling thought, but it is one which a moment's careful consideration will show is the only conceivable explanation of all the facts of physical continuity. Once it is grasped a flood of light is thrown upon the whole science of Embryology. The individual is seen to be literally a “chip of the old block,” and the “old block” means the whole sequence of germ-cells which has preceded his formation. In the light of this fact it is obvious why like produces like; indeed, it is obvious that it must do so. Further, we now understand at once that since one generation of germ-cells directly produces those of the next, there is no reason in the world why an individual should not more nearly resemble a remote ancestor than his own immediate parents. As a simple matter of fact this frequently happens. He does so because the germ-cell from which he sprang is composed of protoplasm handed down in direct continuity by successive generations of germ-cells from time immemorial. In fact the problem in the light of this evidence is not so much—as it always seems to the writer—to understand why children resemble parents and ancestors, as to understand how it is that they do not resemble them more.

There is no difficulty now in explaining the fundamental propositions with which we started, namely, that children resemble their parents. There is no difficulty in the understanding why a child resembles not only its immediate parents, but even its ancestors. There is even no difficulty in understanding why a child should resemble its ancestors, even though it does not resemble its parents. Given the simple truth that the germ-plasm is continuous from one generation to another, all these things become as clear as daylight.

But we also start with another general proposition, namely, that children differ from their parents, and it is this question of the variation in offspring which must now claim our attention for a moment. By this term we mean to convey the fact that although every child has a real resemblance to its parents or its ancestors, it inevitably and invariably shows differences even if these be more minute than the resemblances. In other words the offspring of a human being, though obviously and necessarily, from the continuity of germ-plasm, it must be another human being, is never exactly similar to any other. Now these variations are many of them present from the very beginning, they take their origin in the germ-plasm of the two germ-cells which form the fertilised ovum. They are, that is to say, many of them germinal in origin. These must be carefully distinguished from such characteristics as are afterwards acquired by the child as the result of its adaptation to the environment in which it passes its existence.

It would be beyond the scope of the present work to enter into all the various theories which have been put forward to account for the fact of variation. It will be sufficient for our purpose here if the reader remembers that it is a universal tendency in all living protoplasm to exhibit variations. It is just as universal as is its continuity of likeness. Moreover, in dealing with the highest animals in which the fertilised ovum, from which the embryo springs, is produced by the union of germ-cells from male and female, one may readily understand that the different lines of descent of the male and female germ-cells may well be responsible for the differences exhibited in the offspring. Obviously the fertilised ovum, if it has to give rise to a normal individual, cannot retain all the characteristics which were possibly existent in both the male germ-cells and the female germ-cells. Some of them must be suppressed or got rid of, otherwise there would be too many characters in the resulting offspring. And, as a matter of fact, such a reduction does actually take place in the physical tissue comprising the fertilised cell, and it is probably at this stage that variations take their origin.

Thus, for example, it is quite impossible that two opposing characteristics can both be represented in the fertilised ovum. One of them must be suppressed or thrown out or got rid of in some way or another. For example, the union of the male element and the female element will give rise to an embryo which may be eventually either a male or a female individual, but cannot be both. There were possibilities of it being either the one or the other at the beginning, but since the two possibilities are mutually antagonistic, one or the other must be eliminated. So again, supposing that the colour of the eyes on the one side were brown, and on the other side blue, the possibilities are that the fertilised ovum may give rise to an individual having either blue eyes or brown eyes, but, again, not both.

A variation in offspring then may be regarded as a difference between that offspring and the parents, which is due to some change in the germ-plasm, some difference, that is, between the germ-plasm from which the parent sprang and that from which the next generation arose. Such differences will, of course, be introduced at the time of fertilisation. It is important to keep clearly in mind the difference between a true variation, in the sense that we have just used the term, and a modification which is caused by the varying effects of influences affecting parents and offspring. Unless these two things are kept mentally distinct, much confusion of thought is apt to arise.

The above statement does not necessarily mean that the germ-plasm carried in the sperms and the ova respectively, cannot be affected in any way. Indeed one is forced to the conclusion that such germ-cells must be influenced by the nutrient fluids supplied to them, and by the existence of toxic or poisonous substances in the body of the parent. It is quite conceivable, and indeed inevitable, that the individual embryo resulting from the fusion of such poisoned germ-cells will show modifications, but these, however, are not to be regarded in any sense as true variations, for the simple reason that these modifications do not take their origin in the actual germ-plasm itself, but are simply the result of abnormal stimuli. Such modifications, no matter in what direction they may be, are, of course, not transmissible to the next generation, for the very obvious reason that the germ-cells which are to be concerned with the next generation have been already produced. The germ-plasm itself passes on unchanged in so far as its hereditary possibilities are concerned. We see, therefore, that in order to think clearly on this matter we must limit the meaning of the word variation to such differences or changes in germ-plasm which indicates some real change of an inheritable nature. The term should not be used to apply to a mere passing environment, in which the germ-plasm happens to be, caused by the presence of poisons, or similar factors.

Given then the fact that variations do constantly and inevitably occur in offspring owing to new qualities arising in germ-plasm itself, it is obvious that these variations are either what are termed “spontaneous,” or else they must be due to the action of the surroundings on the germ-plasm. By the term “spontaneous,” in this connection, it is not meant that these variations arise without cause or in a haphazard manner. It is simply meant to imply that the present state of our knowledge does not justify us in stating what does actually cause the variation in germ-plasm, or the laws in accordance with which such variations occur. That they must be a matter of cause and effect and law every biologist believes, but until the law can be demonstrated, the term “spontaneous” may well be retained to distinguish these variations from those which arise by the obvious action of environment. For example, the variations which occur as the results of reproduction from two parents, do so because of the mingling of the respective germ-cells, and such variations come under the group of “spontaneous”; whereas changes induced on account of the food supplied, or poisonous substances in fluids surrounding germ-cells are not spontaneous, but environmental.