uniform in appearance and in size, each with its little mass of protoplasm about it, and this protoplasm appearing in all the cells under microscopic examination very much the same. We can not in this stage of development say of a given cell that it represents any special structure, by which, if we saw it isolated under the microscope, we could determine from what part of the young embryonic body it was derived. When we see a cell from the adult we can determine its origin with certainty by its microscopic appearance alone. As development progresses, the simple condition of the cells is gradually obliterated, but we find another condition arising which we call the differentiated one. Differentiation is a process which goes on in the body as a whole, but of course it is also a function of each individual cell. We can see something of the process of differentiation if we study the unicellular organisms, those creatures, each of which is complete in itself, although it consists of but a single cell, not of countless millions of cells as we do. The picture (Fig. 41) which I have chosen to throw upon the screen is one which I think might have an additional interest to you, for it is a photograph from the living cell known as the parasite producing dysentery. Its scientific name is amœba coli. It is a photograph from life. Here vaguely in the center, marked with a finer granulation, and some of the darker spots in it, we can distinguish the nucleus: here is the outline of the protoplasm of this cell. and in it are included some particles of food which this protoplasmic body has absorbed for purposes of digestion. This is a unicellular parasitic organism with scarcely any differentiation of its structure. The next of the slides shows us again another of these parasitic simple organisms. The figure here to the right of the field of view is the one which should especially attract your attention. The other two bodies near it are blood corpuscles, human blood corpuscles. The organism in this case is the one which causes malarial fever, and it is in a particular stage of its development; that which we distinguish as the tertian malarial parasite is the one here represented. You can see in this case also the outline of the nucleus, surrounded by the protoplasm—the whole thing only a little bigger than a single human blood corpuscle. Here also we note the absence of differentiation. Another stage of this same tertian malarial parasite is shown next. This I have projected upon the screen because it illustrates more clearly than the other the nucleus and the small amount of protoplasm about the nucleus. The malarial organism is one of great vitality, capable of enormously rapid multiplication, and it undoubtedly owes that faculty to its constitution, to the relation between the nucleus and the protoplasm. I will now show you another picture of parasites—one form of which, in a related species, occurs in man. This particular form is one which occurs in the rat and is called the Trypanosoma. You can see that the body, instead of being a small and simple struc-
