While the increase of the protoplasm is going on, we find that there is an advance in the structure, in the differentiation. Now you may recall what I have mentioned earlier in this lecture, the further fundamental fact that the loss in the rate of growth is greatest in the young, least in the old, and that as we go back from old age towards youth, and then into the embryonic period, we find an ever-increasing power of growth, but that it is during the embryonic period that the loss of the power of growth is greatest. It is to the embryonic period, therefore, that I have turned in order to ascertain whether the rate of differentiation shows a similar relation in the development of the organism.
We have a large series of microscopic preparations of rabbit embryos in the embryological laboratory of the Harvard Medical School. Utilizing these, I found that at seven or eight days of development there is scarcely a trace of differentiation. The cells are in the condition of those which I showed to you earlier in the lecture upon the screen. At sixteen and a half days, a stage of development of which I have some good preparations, I found that a great deal had been accomplished. At seven days there was no brain, there was no spinal cord, nothing that could possibly be called skin or muscle, or intestine or heart. None of those things were yet produced. But at sixteen and one half—in other words, after a very brief period indeed—only nine days of the whole life of the animal—there have arisen from this inchoate beginning all the principal organs of the body. The brain is there, divided up into its principal fundamental parts; the spinal cord has its nerves in connection with the various parts of the body; there is a trace of the skeletal element: the stomach, the liver, the pancreas, the intestines, are all present and well defined; the heart is a large and beating organ, amply supplied with blood, connected with vessels, which carry out and bring back the blood and are all far along in their development. Equally instructive is the microscopic examination, for we can see that the cells themselves have been changed. Not only have the great organs been mapped out in this brief period, but the cells which belong to them have for each organ acquired a characteristic quality. In the brain there are nerve cells with their long processes to carry the impulse in; the single process (axon) to carry it out. The glands in the stomach have the cells which are to build them already there. The muscles which are to move the stomach are beginning to appear as cells of a special form. Nerve fibers extend down into the gastric region and to the various distant organs of the body. Muscle fibers can l)e recognized along the back and in the limbs, and so in every part of the body we can detect cells already far advanced in their development. It is not certainly too much to say that in the brief period of these nine days fully as much differentiation has been accomplished as is accomplished during the entire remainder of the life of the animal. We do not, at present at least, possess any method of measuring dif-