Chemical Relations of Nucleus and Cytoplasm

CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM It is no part of the purpose of this work to give even a sketch of general cell-chemistry. I shall only attempt to consider certain questions that bear directly upon the functional relations of nucleus and cytoplasm and are of especial interest in relation to the process of nutrition and through it to the problems of development. It has often been pointed out that we know little or nothing of the chemical conditions existing in living protoplasm, since every attempt to examine them by precise methods necessarily kills the protoplasm. We must, therefore, in the main rest content with inferences based upon the chemical behaviour of dead cells. But even here investigation is beset with difficulties, since it is in most cases impossible to isolate the various parts of the cell for accurate chemical analysis, and we are obliged to rely largely on the less precise method of observing with the microscope the visible effects of dyes and other reagents. This difficulty is increased by the fact that both cytoplasm and karyoplasm are not simple chemical compounds, but mixtures of many complex substances ; and both, moreover, undergo periodic changes of a complicated character which differ very widely in different kinds of cells. Our knowledge is, therefore, still fragmentary, and we have as yet scarcely passed the threshold of a subject which belongs largely to the cytology of the future.

It has been shown in the foregoing chapter that all the parts of the cell arise as local differentiations of an all-pervading substratum which in the greater number of cases, perhaps in all, has the form of a sponge-like network. Cell-organs, such as the nucleus, the spindle and asters, the centrosome, are to be regarded as specialized areas in this network, just as the visible organs of the multicellular body are specialized regions in the all-pervading cellular tissue. And precisely as the various organs and tissues are the seat of special chemical activities leading to the formation and characteristic transformation of specific substances, — as for instance haemoglobin is characteristic of the red blood-corpuscles, or chlorophyll of the assimilating tissues of plants, — so in the cell the various morphological regions are areas of specific chemical activities and are characterized by the presence of corresponding substances. The morphological differentiation of cell-organs is therefore in a way the visible expression of underlying chemical specializations ; and these are in the last analysis reducible to differences of metabolic action.

The Proteids and their Allies

The most important chemical compounds found in the cell are the group of protein substances; and there is every reason to believe that these form the principal basis of living protoplasm in all of its forms. These substances are complex compounds of carbon, hydrogen, nitrogen, and oxygen, often containing a small percentage of sulphur, and in some cases also phosphorus and iron. They form a very extensive group of which the different members differ considerably in physical and chemical properties, though all have certain common traits and are closely related. They are variously classified even by the latest writers. Halliburton ('93) employs the word " proteids " as synonymous with albuminous substances, including under them the various forms of albumin (egg-albumin, cell-albumin, muscle-albumin, vegetable-albumins), globulin (fibrinogen, vitellin, etc.), and the peptones (diffusible hydrated proteids). This author places in a separate class of albuminoids another series of nearly related substances (reckoned by some chemists among the " proteids "), examples of which are gelatine, mucin, and especially nuclein, and the nucleo-albumins. The three last-named bodies are characterized by the presence of phosphorus, in which respect they show a very definite contrast to the " proteids," many of which, such as egg-albumin, contain no phosphorus, and others only a trace. By Hammarsten and some others the word " proteid " is, however, employed in a more restricted sense, being applied to substances such as the nucleins and nucleo-proteids, of greater complexity than the albumins and globulins. The latter, together with the nucleo-albumins, are classed as albuminous bodies (Eiweisskorper).1 The distribution of these substances throughout the cell varies greatly not only in different cells, but at different periods in the life of the same cell. The cardinal fact always, however, remains, that there is a definite and constant contrast between nucleus and cytoplasm. The latter always contains large quantities of nucleo-albumins, certain globulins, and sometimes small quantities of albumins and peptones ; the former contains, in addition to these, nuclein and nucleo-proteids, which as the names indicate, forms its main bulk and its most constant and characteristic feature. It is the remarkable substance, nuclein, — which is almost certainly identical with chromatin, — that chiefly claims our attention here on account of the physiological role of the nucleus.