the cells, but a more careful study of the history of the connective tissue has revealed the very interesting and instructive fact that the fibrils, now separate from the cells, arose by a metamorphosis of the protoplasm of the cells—that they are first formed out of some of the protoplasm of these cells, then split off from them, and come to lie in the intercellular regions, so that here we have another type of cell differentiation brought to our notice, one in which the product is separated from the parent body to which it owes its origin. Now you will perceive immediately, if you recall the series of pictures which have just passed before us on the screen, very great differences in the types of differentiation which occur in the body, and had we time we might find a very much larger range easily to be represented before us.
In the second lecture a picture was projected upon the screen, which showed motor nerve cells of various animals. You will recall that I directed your attention to the fact that the largest animal, the elephant, has the largest cells, and the smallest animals, the rat, the mouse and the little bat, have the smallest ones. But let me point out to you that the question of the size of cells is exceeding complex, and that in studying it we have to exercise a great deal of caution. We know that, with the exception of the nerve cells and to a minor degree with the exception of the muscle fibers, the cells in each animal are more or less uniform constants in size. The cells of different organs differ somewhat from one another. A single organ may have in its different parts typical sizes of cells, but each of these kinds of cells has its definite dimensions. When one animal is larger than another,
