nerve cells by which the higher functions of the body are carried on. They represent the extreme of cellular differentiation, and almost never do we see these cells multiplying after the differentiation is accomplished. Presented in this form, we then recognize, it seems to me clearly, the effect of differentiation upon the growth of cells. The facts are clear as to their meaning.
We can, however, proceed a little farther than this, because we can actually determine, approximately at least, the rate at which cells multiply, and that we can do by means of determining the mitotic index. The mitotic index is the number of cells to be found at any given moment in the active process of division out of a total of one thousand cells.
May I pause a moment to recall this picture to you and ask you to notice at this point the curious darker spot which represents a nucleus in process of division? You will see it would be easy in such a preparation as this to count 
Fig. 61. Portion of the Outer Wall of a Primitive Muscular Segment of a Cat Embryo of 4.6 mm. Harvard Embryological Collection Series 398, section 115. The resting nuclei are oval, pale and granular. The dividing or mitotic nuclei, of which there are three, are dark, irregular in outline and show the chromosomes. In this case the dividing nuclei all lie near the inner surface of the wall. The picture illustrates the ease with which mitotic figures may be recognized. the nuclei one by one until one had got up to a thousand, and to record, as one went along, how many of the nuclei are in process of division, for the nucleus in division is easily recognized. This process of division is named mitosis: the figure which the nucleus presents while it is undergoing division we call a mitotic figure. Counting the dividing nuclei, we may determine that in a thousand cells there are a given number which have nuclei in process of division, and such a number I propose to call "the mitotic index." I wish now only to call to you attention this picture because it enables me to illustrate before you the method of measuring the mitotic index.
In the rabbit embryo at seven and one half days, I have found by actual count that there are in the outer layer of cells, known technically as the ectoderm, 18 of these divisions per thousand. In the middle layer, technically the mesoderm, 17, and in the inner layer, the entoderm, 18. At ten days we find the number already reduced, and the figures are, respectively, 14, 13 and 15, and for the cells of the blood only 10. There has already been a great reduction. In the next phase of development (rabbit embryo of thirteen days), we find, however, that the parts are growing irregularly, some faster, some slower. We note that wherever a trace of differentiation has occurred, the rate of growth is diminished: where that differentiation does not show itself, the
