Schwann’s most important conclusion from this universal presence of soft corpuscles of cell-substance, each with its globular nucleus, in all the tissues and most varied parts of animals as well as plants, was that the life of a living thing, the chemical and physical changes which go on in it from birth to death, consist in chemical and physical changes in each of these microscopic, nucleated bodies, and that the life of the whole animal or plant is the sum of the lives of these microscopic units. If we wish to know more about the real nature of the growth and activities of living things, said Schwann, we must thoroughly study and ascertain the chemical and physical changes, and the properties of the cell-substance in all the different varieties of tissue. That is the celebrated “cell-theory” of Schwann. And this examination of, and experiment with, the cells of all kinds of tissues of plants and animals has been going on ever since Schwann made his historic statement more than seventy years ago. The branch of science called “histology” is the outcome of that study. Microscopes have been immensely improved since Schwann wrote, first in England by the father of the present Lord Lister, then later in Germany by Abbé and Zeiss, of Jena. A variety of methods have been devised for making the “cells” in thick, solid tissues visible. Very thin sections—thin enough to be transparent—were at first cut from the fresh tissues, and examined by transmitted light. This did very well in a rough way, but better results were obtained by hardening the tissues in alcohol or chromic acid, when wonderfully fine sections could be cut and rendered translucent by soaking in varnish, in which they were preserved for study with the microscope, between two plates of glass. The sections were stained with various dyes, such as carmine, log-wood, the aniline dyes, etc., and it was found that the nuclei of the cells and the granules and fibres both in the minute cells and in the surrounding substance manufactured by them, could be distinguished more clearly by means of their differing affinity for the dyes. And whilst endless section-cutting and staining and careful drawing and record of the structure discovered, was proceeding in hundreds of laboratories—other observers especially devoted themselves to the difficult task of seeing the cell-substance or protoplasm and its nucleus under the highest power of the microscope, whilst still alive! It would seem a hopeless task to examine with a high-power microscope the cells (less than a thousandth of an inch broad) inside the solid stem or leaves of a plant or of an animal’s body without killing the plant or animal and the cells of which they consist. As most of my readers know, the front lens (or “glass”) of a high-power microscope has to be brought very close indeed to any object in order to bring it into focus—as near as the one twenty-fifth of an inch. Then the object