Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/866

816 Begin- of Bichat (1771-1802), who distinguished by naked-eye nings of characters the different structural materials of which the organs of man and the higher animals are built, and thus founded in first outline the science of histology. By the end of the first quarter of this century it had become clear to the minds of the anatomico- physiological students of animal life that the animal body was subject to the same physical laws as other matter, although it was still held that some additional and mysterious agent so-called " vitality " was at work in living bodies. It had become clear that animal material could be investigated chemically, and that the processes of digestion, assimilation, respira tion, and secretion were chemical processes. Scliwann s To a considerable extent the chemical composition and properties of the tissues, and the chemical nature of the varioxis changes of life and of putrefaction after death, had been investigated, but one step was yet to be taken which brings the study of ultimate structure, chemical activity, form, and the formation of form to a single focus. This was taken by Theodore Schwann (1810-1881), who in 1839 published his epoch-making cell-theory. Schwann was a pupil of Johann Mliller, and there can be little doubt that the ideas of the pupil are to be credited in some measure to the master. Schwann took up the thread of microscopic investigation which had been sedu lously pursued by a distinct line of students since the days of Hook and Leeuwenhoek, and had resulted in a general doctrine among botanists of the cellular struc ture of all the parts of plants. Schwann showed not only that plants are uniformly built up by these corpuscular units (of which Robert Browne in 1833 had described the peculiar nucleated structure), but that all animal tissues are also so built up. That, however, was not Scliwann s chief point. The cell -theory for which he is famous is this, that the substance of the individual cell is the seat of those chemical processes which seen en masse we call life, and that the differences in the properties of the differ ent tissues and organs of animals and plants depend on a difference in the chemical and physical activity of the con stituent cells, resulting in a difference in the form of the cells and in a concomitant difference of activity. Schwann thus pointed to the microscopic cell-unit as the thing to be studied in order to arrive at a true knowledge of the processes of life and the significance of form. In founding the study of cell-substance (or protoplasm, as it was subse quently called by Max Schultze in 1861, adopting the name used by botanists for vegetable cell-contents) Schwann united two lines of inquiry, viz., that of minute investi gation of structure and development and that of zoo- chemistry and zoo -physics. He spent a large part of the next forty years in an attempt to penetrate further into the structure of cell-substance ; he hoped to be able to find in cell-substance ultimate visible molecules, a knowledge of the arrangement and characters of which would explain the varying properties of protoplasm. Schwann s It is not a little remarkable that Schwann, who thus brought about the union of physiological and morphological study by his conception of cell-substance, should also have been the initiator of that special kind of experimental in vestigation of the physical properties of tissues by the exact methods used by physicists which, by the aid of the kymo- graphion, the thermo-electric pile, and the galvanometer, has been so largely pursued during the last thirty years in our physiological laboratories. It is perhaps less surprising that Schwann, who had so vivid a conception of the activity and potentialities of the cell-unit, should have been the discoverer of the immensely important fact that putrefaction and fermentation arc not the conse quences of death but of life, and that without the presence of living Bacteria putrefaction does not occur, whilst he also is the discoverer of the fact that the yeast which causes alcoholic fermentation is a mass of unicellular living organisms. From Schwann s time onward the cell became more and Develo more the point of observation and experiment in the pro- meut gress of both morphography and physiology. It was soon ?f " , shown, chiefly through Kolliker and Remak, that all cells originate by fission from pre-existing cells, a fact un known to Schwann, and the doctrine "omnis cellula e cellula " was established. It was also demonstrated that the Mammalian egg discovered by Von Baer was a typical nucleated cell, and that all animals, and plants also (this generalization took thirty years to establish), take their origin from an egg, which is in essence and in fact a single nucleated cell. The doctrine of Harvey, "omne vivum ex ovo," thus received its most ample justification. The study of " growth from the egg " became necessarily a study of the multiplication by fission of the egg -cell and its fission-products, their arrangement in layers, and the chemical metamorphosis of their substance and exuda tions. This study, as well as the allied investigation of the cell-structure of the adult tissues, was immensely faci litated by methods of hardening, staining, section-cutting, and clarifying which grew up after Schwann s time, and have their present highest development in the automatic microtome of Caldwell, which can be worked by a motor, and delivers consecutive sections of animal tissues or em bryos j^Wth of an inch thick, arranged in the form of ribbons, ready for examination with the microscope, at the rate of one hundred or more per minute. Strieker of Vienna was the first to embed embryos in waxy material for the purpose of cutting thin sections of them, about twenty- five years ago, and R. Leuckart of Leipsic was subsequently the first to employ this method in the study of the structure of small Invertebrata. The knowledge of the anatomical facts of cellular de velopment and cellular structure necessarily gave im mensely increased precision to the notion of gradation of structure in the animal series from simple to complex, and rendered Darwin s doctrine the more readily accepted. It was not, however, until after Darwin s date (1859) that the existence of unicellular animals was fully admitted, and the general facts of cellular embryology established throughout the animal kingdom. Similarly cellular physiology, by establishing the concep- Cellula tion of a simple optically homogeneous cell-substance as physio the seat of the activities which we call "life," rendered it ogy possible to accept the suggestion of a simple "substance of life" which might have been evolved from simpler non living matter by natural processes depending on physical and chemical laws. It is noteworthy that Darwin himself appears not to have been influenced directly by any such physiological or chemico-physical doctrine as to "proto plasm " or cell-substance. Nevertheless the way was prepared for the reception of Darwin s theory by this state of physiological knowledge. The word " protoplasm " requires a little further notice. Proto- Protoplasm was applied by Von Mohl and by Max Schultze plasm, to the slimy substance of the cell, including therein both the general thinner material and the nucleus. It is, as Roscoe remarked at Manchester (Brit. Ass. Address, 1887), a structure and not a chemical body. Nevertheless gradually physiologists have come to use the word " protoplasm " for one of the chemical substances of which Schultze s proto plasm is a structural mixture namely, that highest point in the chemical elaboration of the molecule which is at tained within the protoplasm, and up to which some of the chemical bodies present are tending, whilst others are degradation products resulting from a downward meta

morphosis of portions of it. This intangible, unstable, all-