The nucleus is a differentiated part of the protoplasm of the cell. Usually it readily coagulates when the cell is injured or killed and then appears as a network of granular fibrils enclosed within a membrane. The importance of the nucleus is most evident dur ing the reproductive activity of the cell which is heralded by the transformation of the nuclear materials into peculiarly constant bodies, the chromosomes. (See CYTOLOGY.) Interchanges between the nucleus and cytoplasm undoubtedly occur, possibly during the entire life cycle of the cell. However, not enough is known at present regarding the nature of this rela tion to permit any certain interpretation. What is known is that a relation does exist which is essential for the continued life of the cell.
Under the microscope the substance of the cytoplasm appears to be a translucent, optically structureless fluid, the hyaloplasm in which lie various sized granules, globules and fibrillae in varying amounts. In many instances these visible bodies are grouped in definite patterns, some of which constitute specialized structures. (See CYTOLOGY.) The significance of these structures must be left to the future.
The presence of visible granules, fibrils and vacuoles in the hyaline matrix is such a universal feature that many of the older conceptions of protoplasm were based on the assumption that these inclusions form an essential part of protoplasmic structure. However, in view of the fact that these structures vary in the protoplasm of different cells, not only in form but also in number, and may be entirely absent in some cells or appear only at some stages in the life of a cell, we must regard them rather as spe cialized differentiations in each case.
A feature which strikes the eye of the observer is the constant movement of the visible granules in the hyaline matrix of the protoplasmic substance. This continual, translational movement, as distinguished from the vibratory Brownian movement, is very noticeable and rapid in some cells and barely perceptible in others.
True Brownian movement is also seen in protoplasm, sometimes in restricted areas and at other times throughout the entire proto plasm of a cell. The protoplasm of. some cells is very fluid, in others it is quite viscous. The viscosity may change at different periods in the life history of a cell.
A factor which has been found to be important in the mainte nance of protoplasm is the antagonistic action of certain salts in the protoplasm and in the fluids which bathe it. As an example we
may discuss the action of two of the most common salts : sodium chloride and calcium chloride. The fluids contained in all living organisms, plant or animal, contain these (or very similar) salts in definite, relative concentrations which approximate those of the same salts in the ocean. Protoplasm will not survive beyond very narrow limits of those relative concentrations, the proportions being about fifty parts of sodium to one of calcium. The actual concentrations of these elements do not appear to be as important as are the proportions in which the two exist. The significance of these salts probably lies in the fact that the salts of sodium have a dispersive action while those of calcium have an aggregative action on two of the most important constituents of protoplasm, viz., proteins and fats.
The chemical state of the proteins and fats probably also accounts for the fact that protoplasm can survive only within narrow limits of an acid-base equilibrium. This is maintained by the buffer action of the acid and basic combinations of certain salts, e.g., the carbonates. In this way any excess acid or base liberated during metabolic activity is neutralized before the proto plasm is affected injuriously. Temperature and oxygen tension in the environment also play important roles in controlling chemical interactions in protoplasm.