H. M. Friedman[146] begins his comprehensive treatise with biological and embryological considerations, and here perhaps he makes his most original suggestions. The higher the plane of the animal, the more marked is cell differentiation or specialization and this affects most cells. Once a degree of differentiation is observed, no backward step to a previous state of generalization, regeneration, or rejuvenation is possible. The higher the ascent of the cell in the plane of differentiation, the lower is its power of rejuvenation. Connective tissue, muscle fiber, and cylindrical cells are the least differentiated and therefore have the greatest power of regeneration. Nerve cells have the least because their work is of a high order and they are most specialized. Nerve fibers are mere conductors and they and epithelial cells have probably the greatest power of regeneration. Again, the more differentiated the cell, the more rapid is its development, early decline, and death. Precocity even of the separate cell purports early maturity. “So senescence is an increased differentiation of the protoplasm, while rejuvenation is an increase of the nuclear elements at the expense of the protoplasm.” The increase of nuclear material allows fission and the formation of new cells. Thus the degree of differentiation is greatest as fission or mytosis is least. The power of regeneration is in direct proportion to the power of cell fission. Thus “the greater the cell differentiation, the smaller the mytotic index.” With maturity the decrease of the mytotic index, or the number of tissues into the composition of which the cell can enter, becomes restricted. The cells in the original germinal layer have before them the possibility of entering into the structure of any tissue, but as cells differentiate the germinal layers take on a more structural character and leave the field to the entrance of cells into different tissue formations more restricted, since during development the number of tissues yet unformed or undifferentiated becomes less and less and once a cell has assumed a personality it must continue to follow it up and cannot diverge from it. This is the law of genetic restriction. The younger the cells, the greater their multiplying power and the greater the tissue possibilities they can choose. Hence morbid tumors are formed from young cells of higher mytotic index whose genetic restriction has not progressed far enough to inhibit range and rapidity of growth. Before genetic restriction young cells may become one tissue or another. Injuries causing cell degeneration of the young cells are often, therefore, the seats of morbid growths. The young or undifferentiated cells forming malignant amorphous tumors and growing in tissues alien to them develop rapidly, probably because they are deprived of the “social” restriction to overgrowth that they would have in their own cell society. Thus the presence of young cells