Biology and Its Makers / With Portraits and Other Illustrations

by William A. Locy

The Discovery of Protoplasm and its Effect on the Cell-Theory.—In 1835, before the announcement of the cell-theory, living matter had been observed by Dujardin. In lower animal forms he noticed a semifluid, jelly-like substance, which he designated sarcode, and which he described as being endowed with all the qualities of life. The same semifluid substance had previously caught the attention of some observers, but no one had as yet announced it as the actual living part of organisms. Schleiden had seen it and called it gum. Dujardin was far from appreciating the full importance of his discovery, and for a long time his description of sarcode remained separate; but in 1846 Hugo von Mohl, a botanist, observed a similar jelly-like substance in plants, which he called plant schleim, and to which he attached the name protoplasma.

The scientific world was now in the position of recognizing living substance, which had been announced as sarcode in lower animals, and as protoplasm in plants; but there was as yet no clear indication that these two substances were practically identical. Gradually there came stealing into the minds of observers the suspicion that the sarcode of the zoölogists and the protoplasm of the botanists were one and the same thing. This proposition was definitely maintained by Cohn in 1850, though with him it was mainly theoretical, since his observations were not sufficiently extensive and accurate to support such a conclusion.

Eleven years later, however, as the result of extended researches, Max Schultze promulgated, in 1861, the protoplasm doctrine, to the effect that the units of organization consist of little masses of protoplasm surrounding a nucleus, and that this protoplasm, or living substance, is practically identical in both plants and animals.

The effect of this conclusion upon the cell-theory was revolutionary. During the time protoplasm was being observed the cell had likewise come under close scrutiny, and naturalists had now an extensive collection of facts upon which to found a theory. It has been shown that many animal cells have no cell-wall, and the final conclusion was inevitable that the essential part of a cell is the semifluid living substance that resides within the cavity when a cell-wall is present. Moreover, when the cell-wall is absent, the protoplasm is the "cell." The position of the nucleus was also determined to be within the living substance, and not, as Schleiden had maintained, within the cell-wall. The definition of Max Schultze, that a cell is a globule of protoplasm surrounding a nucleus, marks a new era in the cell-theory, in which the original generalization became consolidated with the protoplasm doctrine.

Further Modifications of the Cell-Theory.—The reformed cell-theory was, however, destined to undergo further modification, and to become greatly extended in its application. At first the cell was regarded merely as an element of structure; then, as a supplement to this restricted view, came the recognition that it is also a unit of physiology, viz., that all physiological activities take place within the cell. Matters did not come to a rest, however, with the recognition of these two fundamental aspects of the cell. The importance of the cell in development also took firmer hold upon the minds of anatomists after it was made clear that both the egg and its fertilizing agents are modified cells of the parent's body. It was necessary to comprehend this fact in order to get a clear idea of the origin of cells within the body of a multicellular organism, and of the relation between the primordial element and the fully developed tissues. Finally, when observers found within the nucleus the bearers of hereditary qualities, they began to realize that a careful study of the behavior of the cell elements during development is necessary for the investigation of hereditary transmissions.

A statement of the cell-theory at the present time, then, must include these four conceptions: the cell as a unit of structure, the cell as a unit of physiological activity, the cell as embracing all hereditary qualities within its substance, and the cell in the historical development of the organism.

Some of these relations may now be more fully illustrated.

Origin of Tissues.—The egg in which all organisms above the very lowest begin, is a single cell having, under the microscope, the appearance shown in Fig. 76. After fertilization, this divides repeatedly, and many cohering cells result. The cells are at first similar, but as they increase in number, and as development proceeds, they grow different, and certain groups are set apart to perform particular duties. The division of physiological labor which arises at this time marks the beginning of separate tissues. It has been demonstrated over and over that all tissues are composed of cells and cell-products, though in some instances they are much modified. The living cells can be seen even in bone and cartilage, in which they are separated by a lifeless matrix, the latter being the product of cellular activity.