development of animal or plant structure. I shall endeavor, therefore, now to give you some insight into the phenomena of cytomorphosis as regarded by the scientific biologist. The first cells which are produced are those which form the young embryo. We speak of them, therefore, as embryonic cells, or cells of the embryonic type. Our next picture illustrates the actual character of such cells as seen with the microscope, for it represents a series of sections through the body of a rabbit embryo, the development of which has lasted only seven and one half days. You will notice at once the simplicity of the structure. There are not yet present any of those parts which we can properly designate as organs. The cells have been produced by their own multiplication and are not yet so numerous but that they could be readily actually counted. They are spread out in somewhat definite layers or sheets, but beyond that they show no definite arrangement which is likely to attract your attention. That which I wish you particularly to observe is that in every part of each of these sections the cells appear very much alike. The nuclei are all similar in character, and for each of them there is more or less protoplasm; but the protoplasm in all parts of these young 
Fig. 6. Portion of a Transverse Section of the Spinal Cord of A Human Embryo of Four Millimeters. Harvard Embryological Collection, series 714. The spinal cord at this stage is a tubular structure. The figure shows a portion of the wall of the tube: the lefthand boundary of the figure corresponds to the inner surface of the tube. rabbits is found to be very similar; and indeed if we should pick out one of these cells and place it by itself under the microscope, it would be impossible to tell what part of the rabbit embryo it had been taken from, so much do all the cells of all the parts resemble one another. We learn from this picture that the embryonic cells are all very much alike, simple in character, have relatively large nuclei, and only a moderate amount of protoplasm for each nucleus to complete the cell.
Very different is the condition of affairs which we find when we turn to the microscopic examination of the adult. Did time permit it would be possible to study a succession of stages and show you that the condition which we are about to study as existing actually in the adult is the result of a gradual progress and that in successive stages of the individual we can find successive stages of cell change; but it will suffice for our immediate purpose to consider the results of differentiation as they are shown to us by the study of the cells of
