240 Rf. ABERCROMBIE nuclear content of the neighboring tissue which contains tlie other portions of the transected nuclei. Only if the average dimension of tlie nuclei in the direction at right angles to that of the section is minute in relation to the thickness of section, will the exaggeration be negligible. The bigger this nuclear dimension is in relation to section thickness, the bigger the error, because the higher the proportion of nuclear fragments. Agdulir ('41) clearly recognized this source of error.

Even if we work in an entirely relative way, and attempt to compare the nuclear populations of different tissues, or of the same tissue under different conditions, we may be led into error. For obviously any difference in size of the nuclei in the tissues compared will by itself lead to different counts. We can put these considerations more exactly as follows. The true density of population, from which can be derived by simple proportion the number of nuclei in any volume of tissue, is for our purposes best regarded as the number of nuclear-points within unit volume. By a nuclear-point I mean any geometrical point of the same relative position in all nu-clei; say midway along the dimension at right angles to the microtome section. A nuclear-point cannot overlap two adjacent sections, and the number of nuclear-points in a section can therefore be extrapolated to the number in any volume of tissue without error, and can serve for exact comparison of nuclear population of different tissues. If P is the average number of nuclear points per section, A is the crude count of number of nuclei seen in the section, R9 is the thickness (in p) of the section, and L the average length (in p) of the nuclei, then