minute drop of human blood can be kept alive for hours, if the glass slide is kept warm, as it easily can be, and I have seen the cells in a drop of frog's blood (skilfully treated) still alive, and exhibiting active movements, a fortnight after the frog, from which the drop of blood came, was dead and buried. These floating, moving

cells of the blood are the "phagocytes," which engulf and digest disease germs and other particles (Fig. 36 B). Other more numerous cells of the blood are the oxygencarriers, or red corpuscles, which do not show any movements or changes of an active kind whilst alive.

THE

XIX

PROTOPLASM, LIFE AND DEATH

HE result of the study of living cell-substance, or protoplasm, is to show that every cell has an individual life, and often makes this manifest by its movement, change of shape, and internal currents of granules, as well as by the special chemical substances it produces and consumes. All depend for their activity upon the presence of free oxygen; all are killed by heat far less than that of boiling water; they continually imbibe water charged with the chemical substances which nourish them and cause them to grow in bulk and to divide into two; and they manufacture various chemical bodies in the protoplasm and emit heat, electrical discharges, and sometimes light. Some or other of them, in fact, do in their small microscopic way all that the complex, big animal or plant, of which they are constituents, is seen to do. The cells of the liver manufacture the bile, those of the salivary glands the saliva, and those of the intestinal wall a mucous fluid, and squeeze out or eject those products into the adjacent ducts (see Fig. 40 C). Other cells lay down (as cell-wall or coating) fibrous and hard substances which form the skeleton; others become converted into horn and are shed from the surface of the skin in man as "scurf"; others form the great contractile masses called muscles. One lot are told off to control

the other cells by something resembling a system of electrical wires and batteries-these are the nerve-cells (Fig. 37 D), with their fine, thread-like branches, the nerve-fibres, which are long enough to permeate every part of the body and place it in connection with the nerve-cells in the great centres called brain, spinal cord, and ganglia.

At one time it was thought that the cells in the tissues of plants and animals could originate de novo by a sort of precipitation of liquid matter.. But it is now known that every cell has originated by the division of a pre-existing cell into two, the nucleus of the mother cell first dividing and then the rest of the cell. "Every cell originates by the fission of a preceding cell" is the law, and to that is added, "Every individual organism, plant or animal, itself originates from a single cell, the fertilised germ-cell." These are two laws of fundamental importance in the study of living things. They are true of man as well as of the smallest worm; of the biggest tree as well as of the most insignificant moss or water-weed. When the fertilised egg-cell divides, and its progeny keep on dividing and growing in bulk by the conversion of nutriment into protoplasm, the dividing cells do not necessarily become entirely nipped off from one another. large tracts of cells (or tissues) we often find that the neighbouring cells are connected to one another by excessively fine filaments of protoplasm. Only twenty years ago it was supposed, whilst the neighbouring cells were thus connected as a rule in animals, as well as being often connected to the finest nerve-filaments, yet that in plants the firm, box-like cases which surround the protoplasmand when seen dried and empty by Robert Hook led him to introduce the word "cell" to describe them-form completely shut cases, so that the living protoplasm of each plant-cell is entirely cut off from its neighbour.

In

This has now been found by improved methods of microscopic examination to be a mistake. The cell-wall in a great many plants, though so firm and cleanly cut in appearance, is yet perforated by fine threads of the cell protoplasm, so that each cell is in living communication with its neighbour. Thus, in plants as well as in animals, the individual cell-units form a more or less continuous whole of living matter, separated by dead, inert cellwalls and products of cell activity; but, nevertheless, connected in definite tracts and regions to one another by continuity of the living matter in the form of excessively fine threads.

Those animals and plants which are built up of many cells of many varieties-that is to say, all but the microscopic unicellular kinds-may be considered as composite organisms-cell-states or communities in which the individual cells, all derived from one original mother-cell, are the citizens, living in groups and habitations (tissues), having their different occupations and capacities, carrying on distinct operations and working together for the common good, the "life," as we call it, of the individual plant or animal which they constitute. This comparison

should serve merely as an illustration of the individual character and co-ordinated activity of the cells of a manycelled plant or animal. It must not be forgotten that the separate cells are all derived by binary division from the original germ-cell, that they have not come into juxtaposition from distinct sources, but often are held together by threads of their living material, which remain after the process of division of one cell into two.

Protoplasm has been called "the physical basis of life." Since the activities to which we give the name "life" reside in protoplasm, and are chemical and physical activities like those of other bodies, even though more subtle and complicated-we are justified in regard

ing protoplasm as the substance in us and other organisms which "lives." Death consists in the destruction-the chemical undoing or decomposition of protoplasm.1 In simple microscopic unicellular animals and plants, this is obvious-so long as the protoplasm retains its chemical structure it is not "dead." Thus, it is possible with many small simple organisms-such as animalcules and the seeds of plants-to dry them, and to expose them to extreme cold, and to deprive them (by aid of a vacuum pump) of all access of free oxygen or other gases. All chemical change is thus necessarily arrested. But the atomic structure of the chemical molecules in the protoplasm is not destroyed. Sir James Dewar, M. Becquerel, and others have shown this by most carefully conducted experiments. Seeds of clover, mustard, and wheat so treated do not "die"; the mechanism remains intact, and when, after many weeks, the seeds are moistened, warmed, and admitted to contact with the atmosphere, the mechanism again begins to work, the protoplasm resumes its activity, the seed "sprouts." Similarly Dewar has shown that bacteria are not killed by extreme cold, the temperature of liquid hydrogen. When thus frozen they remain inert-but are even in this condition liable to be "killed" by exposure to the blue and ultra-blue rays of sunlight! Life was defined by Herbert Spencer as "the 'continuous' adjustment of internal to external relations,"

1 Protoplasm is not a single chemical compound; it is the name given to the soft, slimy substance of cells, and contains many chemical compounds-proteids, fats, and others; some on the way to assume greater chemical complexity; others in process of destruction. The critical highest chemical body concealed in protoplasm has no generally recognised name. It is a proteid-like body, consisting chiefly of carbon, oxygen, hydrogen, and nitrogen, with some saline constituents. This is the real ultimate "living matter," and I suggested in the Encyclopedia Britannica (article Protozoa) in 1886 that it should be called "plasmogen."