THE NATURE OF CELL-ORGANS The cell is, in Brucke's words, an elementary organism, which may by itself perform all the characteristic operations of life, as is the case with the unicellular organisms, and in a sense also with the germcells. Even when the cell is but a constituent unit of a higher grade of organization, as in multicellular forms, it is no less truly an organism, and in a measure leads an independent life, even though its functions be restricted and subordinated to the common life. It is true that the earlier conception of the multicellular body as a colony of one-celled forms cannot be accepted without certain reservations.' Nevertheless, all the facts at our command indicate that the tissuecell possesses the same morphological organization as the egg-cell, or the protozoan, and the same fundamental physiological properties as well. Like these the tissue-cell has its differentiated structural parts or organs, and we have now to inquire how these cell-organs are to be conceived.
The visible organs of the cell fall under two categories according as they are merely temporary structures, formed anew in each successive cell-generation out of the common structural basis, or permanent structures whose identity is never lost since they are directly handed on by division from cell to cell. To the former category belong, in general, such structures as cilia, pseudopodia, and the like ; to the latter, the nucleus, probably also the centrosome, and the plastids of plant-cells. A peculiar interest attaches to the permanent cell-organs. Closely inter-related as these organs are, they nevertheless have a remarkable degree of morphological independence. They assimilate food, grow, divide, and perform their own characteristic actions like coexistent but independent organisms, of a lower grade than the cell, living together in colonial or symbiotic association. So striking is this morphological and physiological autonomy in the case of the green plastids or chromatophores that neither botanists nor zoologists are as yet able to distinguish with absolute certainty between those that form an integral part of the cell, as in the higher green plants, and those that are actually independent organisms living symbiotically within it, as is probably the case with the yellow cells of Radiolaria. Even so acute an investi
gator as Watase ('93, I) has not hesitated to regard the nucleus itself — or rather the chromosome — as a distinct organism living in symbiotic association with the cytoplasm, but having had, in an historical sense, a different origin. It is but a short step from this conelusion to the view that the centrosome, too, is such an independent organism and that the cell is a symbiotic association of at least three dissimilar living beings ! Such a conception would, however, as I believe, be in the highest degree misleading, even if with Watase we limit it to the nucleus and the cytoplasm. The facts point rather to the conclusion that all cell-organs arise as differentiated areas in the common structural basis of the cell, and that their morphological character is the outward expression of localized and specific forms of metabolic activity.
It is certain that some of the cell-organs are the seat of specific chemical changes. Chromatin (nuclein) is formed only in the nucleus. The various forms of plastids have specific metabolic powers, giving rise to chlorophyll, to pigment, or to starch, according to their nature. The centrosome, as BUtschli, Strasburger, and Heidenhain have insisted, possesses a specific chemical character to which its remarkable effect on the cytoplasm must be due.' Even in regions of the cytoplasm not differentiated into distinct cell-organs the metabolic activities may show specific and constant localization, as shown by the deposit of zymogen-granules, the secretion of membranes, the formation of muscle-fibres, and a multitude of related facts. Physiologically, therefore, no line of demarcation can be drawn between permanent cell-organs, transient cell-organs, and areas of the cell-substance that are physiologically specialized but not yet morphologically differentiated into organs. When we turn to the structural relations of cellorgans, we find, I think, reason to accept the same conclusion in a morphological sense. The subject may best be approached by a consideration of the structural basis of the cell and the morphological relations between nucleus and cytoplasm.