tion in that they differ according to the size of the animal; and their nuclei differ also, for as the cells become big the nuclei grow likewise. Here are nerve-cell nuclei in the rabbit of twelve and one half days not differing in their dimensions essentially from the nuclei of other types, but in the two lower figures, 22 and 23, we see nuclei corresponding to the cells of the rabbit at sixteen and one half days. These cells have begun to enlarge, to assume the greater dimensions of the nerve cells, which is characteristic of the rabbit; and accompanying the enlargement of the cells there has been an expansion of the nuclei also. But this does not affect, as you will readily see by the pictures upon the screen, the nuclei of any other sort of tissue, the nuclei of any other organ of the body.
"We must therefore add to our conceptions in regard to the relations of the nucleus and protoplasm, as quantitatively expressed, this further notion that there is during the early period of development an actual reduction in the size of the nucleus. When this reduction has taken place it is of course evident to any one at all acquainted with the principles of cytology that the cells are in a very different state from what they were in before. They are no longer such cells as they were when the nucleus was large, and the nuclei in the different parts of the body alike in character. Here the relations are fundamentally changed. We do not find that these nuclei ever get back from the complex variety of organization which they present to us in later stages to the earlier condition when they were all alike; yet only with cells of this uniform sort can development begin. We should therefore, if we reasoned only from the data which I have thus far presented to you, come to the conclusion that reproduction would be impossible, that the cells of the body, having been so changed, as we have seen, are no longer capable of returning backwards along the path they have journeyed; they can only remain where they are, or go yet further onward in the career of cytomorphosis. Nature, however, has met this difficulty by a way which we have only recently discovered. We are not yet sure that the way we have discovered is the only way, that it is the universal method in the case of all animals for accomplishing the purpose. The discovery of this method of providing for the perpetuation of youthfulness from one generation to another, the youthfulness of the cell of man, is due to the investigations of Professor Nussbaum, of Bonn. The theory, which he put forward, has been verified by subsequent examinations and investigation, and confirmed, I am glad to say, in part by some very interesting and careful observations which have been made here in Boston. Perhaps the very best confirmation of all is the recent extension of our knowledge in regard to this theory which comes from the investigations of Dr. B. M. Allen, made at Madison, on the process as we find it in the developing turtle. It is really essentially a very simple tiling. Nature seems to take some of the cells which are in the
