ABSTRACT
WHATEVER may have been the stage of earth-growth at which the forces which make life first took form, it seems not unreasonable to suppose that the material upon which they acted ultimately responded by grouping itself into what (for want of a better acquaintance with its properties) has been called protoplasm, or first-material. The most elementary form of life at present known is a single cell: it is definitely separable from its surroundings and has an appreciable internal content. Whether it has an envelope sufficiently clearly marked to be called a skin is not clear: it would perhaps be more accurate to compare its outside surface to that of water standing up beyond the top of an overfilled glass: whatever its nature may be, it is known as its external membrane. It has then what the ordinary man would call an inside and an outside, and it is customary to regard it as the most simple life-form. Examples of it may be found in the apparently green growth which appears on the sunlit surface of standing water (on the contact-horizon of water and air). The desire to see it more closely suggests the use of a microscope. Now the difficulties inherent in the use of a microscope upon living cells are many and serious: they are well stated by a practised observer (Bayliss 1 ) of unrivalled experience. “It is unfortunate that the study of the phenomena presented by living cells is rendered difficult by the fact that so little can be seen by microscopic examination.” (Here one has to remember that the sense organ upon which one is depending for receiving an external stimulus is the eye, and that the vibrations received are those of light, reflected from the surface of the object being examined: the appearance of a mother-of-pearl handle of a knife is a good example.) “The presence of different structures in a cell, even supposing that they are colourless, can be detected if they have refractive indices differing from that of the surrounding substance.... Now most of the various structures in living cells possess very nearly the same refractive index, a fact which renders this mode of microscopic vision of limited use.” It is however possible to stain the substance under examination with certain dyes: this is a regular and standardised technique, under which definite substances react in definite ways, and are even named from their method of reaction. Some of the dyes are unavoidably poisonous and injure or destroy the life of the cell: others produce in the cell structures due to the dye (compare the production of pearl in oysters by introducing foreign matter). “After fixation, neither the form nor the staining properties give correct information as to the relationship of the constituents of the original system.” That is as much as to say that without staining the structures are to some extent invisible, and that with staining there is no guarantee that the normal life of the cell can be observed.
