rate of growth may even increase in order to acquire a certain special development of a particular part. So that instead of uniformity of values for the mitotic index, we get a great variety. But, nevertheless, the general decline can be demonstrated by the figures. In the spinal cord the index is 11, in the general connective tissue of the body 10; for the cells of the liver 11; in the outside layer of the skin 10; in the excretory organ 6; in the tissue which forms the center of the limb also 6. There has, then, been a rapid decline in the rate of cell multiplication just in this period when differentiation is going on. This is, so far as I know, an entirely new line of research. The counting of a thousand cells is not a thing to be done very rapidly; it must be undertaken with patience, care, and requires time. It has not, I regret to say, been possible for me yet to extend the number of these counts beyond those I have given you, but it is easy to say that in the yet more differentiated state, the number of cells in division is constantly lessened, and it is only a question of counting to determine the mitotic index accurately. That there is a further diminution beyond that which the mitotic indices I have demonstrated to you represent is perfectly certain, I only regret that I am not able to give you exact numerical values.
I wish very much that my time permitted me to branch off into certain topics Intimately associated with the general theme we have been considering together on these successive evenings, but we can only allude to a few of these. The first collateral subject on which I wish to speak to you briefly is that which we call the law of genetic restriction, which means that after a cell has progressed and is differentiated a certain distance, its fate is by so much determined. It may from that pass on, turn in one direction or another, always progressing, going onward in its cytomorphosis; but the general direction has been prescribed, and the possibilities of that cell as it progresses in its development become more and more restricted. For instance, the cells which are set apart to form the central nervous system after they are so set apart can not form any other kind of tissue. After the nervous system is separated in the progress of development from the rest of the body, its cells may become either nerve cells proper or supporting cells (neuroglia), which latter never acquire the nervous character proper, but serve to uphold and keep in place the true nervous elements. They represent the skeleton of the central nervous system. After the cells of the nervous system are separated into these two fundamental classes they can not change. A cell forming a part of the supporting framework of the brain can not become a nerve cell; and a nerve cell can not become a supporting cell. The destiny of them becomes more and more fixed, their future possibilities more and more limited, as their cytomorphosis goes on.
The law of genetic restriction has a very important bearing upon questions of disease. When disease occurs, the cells of the body offer
