change in their number, kind and arrangement.  Atom groups as decomposition products are constantly given off from it, and in return it receives from without other atom groups with which it regenerates its substance or increases in amount.  All definitions of life convey this idea of activity.  Herbert Spencer says, “Life is the continuous adjustment of internal relations to external conditions.”  The molecules of the substances forming the living material are large, complex and unstable, and as such they constantly tend to pass from the complex to the simple, from unstable to stable equilibrium.  The elementary substances which form living material are known, but it has hitherto not been found possible artificially so to combine these substances that the resulting mass will exhibit those activities which we call the phenomena of life.  The distinction between living and nonliving matter is manifest only when the sum of the activities of the living matter is considered; any single phenomenon of the living may appear also in the non-living material.  Probably the most distinguishing criterion of living matter is found in its individuality, which undoubtedly depends upon differences in structure, whether physical or chemical, between the different units.

Certain conditions are essential for the continued existence of living matter.  It must be surrounded by a fluid or semi-fluid medium in order that there may be easy interchange with the environment.  It must constantly receive from the outside a supply of energy in the form of food, and substances formed as the result of the intracellular chemical activity must be removed.  In the case of many animals it seems as though the necessity of a fluid environment for living matter did not apply, for the superficial cells of the skin have no fluid around them; these cells, however, are dead, and serve merely a mechanical or protective purpose.  All the living cells of the skin and all the cells beneath this have fluid around them.

Living matter occurs always in the form of small masses called “cells,” which are the living units.  The cells vary in form, structure and size, some being so large that they can be seen with the naked eye, while others are so small that they cannot be distinctly seen with the highest power of the microscope.  The living thing or organism may be composed of a single cell or, in the case of the higher animals and plants, may be formed of great numbers of cells, those of a similar character being combined in masses to form organs such as the liver and brain.

In each cell there is a differentiated area constituting a special structure, the nucleus, which contains a peculiar material called “chromatin.”  The nucleus has chiefly to do with the multiplication of the cell and contains the factors which determine heredity.  The mass outside of the nucleus is termed “cytoplasm,” and this may be homogeneous in appearance or may contain granules.  On the outside there is a more or less definite cell membrane.  It is generally believed that the cell material has a semi-fluid or gelatinous consistency and is contained within an intracellular meshwork.  It is an extraordinarily complex mass, whether regarded from a chemical or physical point of view. (Fig. 1.)