becomes very small indeed, less than 20, so that during the prenatal life of man, as we have seen in the prenatal life of the rabbit and of the chick, the decline in the power of growth is going on steadily all the time.
I shall use the few remaining moments to report to you yet another bit of evidence of the originally enormous power of growth. It has been estimated that the germ of the mammal, with which the development commences, has a weight of 0.6 milligram; another estimate which I have found is of 0.3 milligram.[1] Perhaps I can give you some idea of what this value means by telling you that if the weight of the original germ of a mammal is assumed to be 0.6 milligram, we could, according to the laws of the United States, send 50,000 such germs by letter postage for two cents. It would take 50,000 germs to make the weight of one letter. That perhaps will give you some impression of the extreme minuteness of the primitive germ. In the human species at the end of even a single month it is no longer merely a germ, but a young human being, very immature, of course, in its development, but already very much larger. I doubt—even after all that I have said this evening about the startling figures of growth for the earlier stages,—I doubt if you are prepared for the fact that the growth of the germ up to the end of the first month represents an increase of over a million per cent. How much over a million per cent, we can not calculate accurately, because we do not know accurately the weight of the original germ, but an increase of a million per cent, is not above the true value. Contrast that with anything which occurs in the later periods. What a vast change has happened! What an immense loss has taken place! The rate of this loss is evidently diminishing. The less occurs with great rapidity in the young—less rapidity the older we become. I attempted to convince you in the first and second lectures that that which we called the condition of old age, is merely the culmination of changes which have been going on from the first stage of the germ up to the adult, the old man or woman. All through the life these changes continue. The result is senility. But if, as the phenomena of growth indicate to us so clearly, it be true that the decline is most rapid at first, then we must expect from the study of the very young stages to find a more favorable occasion for analysis of the factors which bring about the loss in the power of growth and change as the final result of which we encounter the senile organism. Not from the study of the old, therefore, but from the study of the very young, of the young embryo, and of the germ, are we to expect insight into the complicated questions which we have begun to consider together. I shall hope in the next lecture to prove to you that the supposition which has guided my own observations is correct, and to be able to show you that we do actually, from the study of the developing embryo, glean some revelations of the cause of old age.
- ↑ These estimates refer to the placental mammals only.
