Theodor Schwann, whose name will therefore ever be honored by all investigators of vital phenomena. What the atom is to the chemist, the cell is to the naturalist. Every cell consists of two essential parts. There is an inner central kernel which is known by the technical name of nucleus, and a covering mass of living material which is termed the protoplasm and constitutes the body of the cell. I will now call for the first of our lantern slides to be thrown upon the screen. It presents to you pictures of the cells as they are found lining the mouth of the European salamander. The two figures at the top illustrate very clearly the elements of the cell. The protoplasm forms a mass, offering in this view no very distinctive characteristics, and therefore offering a somewhat marked contrast with the nucleus which presents in its interior a number of granules and threads. Every nucleus consists of a membrane by which it is separated from the protoplasm, and three internal constituents: First, a network of living material, more or less intermingled with which is a second special substance, chromatin, which owes its name to the very marked affinity which it displays for the various artificial coloring matters which are employed, in microscopical research. The third of the internal nuclear constituents we may call the sap. the fluid material which fills out the meshes of the network. Later on we shall have occasion to study somewhat more carefully the principal variations which nuclei of different kinds may present to us, and we shall learn from such study that we may derive some further insight into the rapidity of development and the nature of the changes which result in old age. While the picture is upon the screen, I wish to call your attention to the other figures which illustrate the process of cell multiplication. As you regard them you will notice in the succession of illustrations that the nucleus has greatly changed its appearance. The substance of the nucleus has gathered into separate granules, each of which is termed a chromosome. These chromosomes are very conspicuous under the microscope, because they absorb artificial stains of many sorts with great avidity and stand out therefore conspicuously colored in our microscopic preparations. They are much more conspicuous than is the substance of the resting nucleus. And this fact, that we can readily distinguish the dividing from the resting nucleus under the microscope, we shall take advantage of later on, for it offers us a means oi investigating the rate of growth in various parts of the body. I should like, therefore, to emphasize the fact at the present time sufficiently to be sure that it will remain in your minds until the later lecture in which we shall make practical use of our acquaintance with it. It is unnecessary for our purposes to enter into a detailed description of the complicated processes of cell division. But let me point out to you that the end result is that where we have one cell we get as the result of division—two; but the
