THE CELL IN RELATION TO THE MULTICELLULAR BODY In analyzing the structure and functions of the individual cell we are accustomed, as a matter of convenience, to regard it as an independent elementary organism or organic unit. Actually, however, it is such an organism only in the case of the unicellular plants and animals and the germ-cells of the multicellular forms. When we consider the tissue-cells of the latter we must take a somewhat different view. As far as structure and origin are concerned the tissuecell is unquestionably of the same morphological value as the one-celled plant or animal ; and in this sense the multicellular body is equivalent to a colony or aggregate of one-celled forms. Physiologically, however, the tissue-cell can only in a limited sense be regarded as an independent unit ; for its autonomy is merged in a greater or less degree into the general life of the organism. From this point of view the tissue-cell must in fact be treated as merely a localized area of activity, provided it is true with the complete apparatus of cell-life, and even capable of independent action within certain limits, yet nevertheless a part and not a whole.
There is at present no biological question of greater moment than the means by which the individual cell-activities are co-ordinated, and the organic unity of the body maintained ; for upon this question hangs not only the problem of the transmission of acquired characters, and the nature of development, but our conception of life itself. Schwann, the father of the cell-theory, very clearly perceived this ; and after an admirably lucid discussion of the facts known to him (1839), drew the conclusion that the life of the organism is essentially a composite ; that each cell has its independent life ; and that "the whole organism subsists only by means of the reciprocal action of the single elementary parts." This conclusion, afterwards elaborated by Virchow and Hackel to the theory of the "cell-state," took a very strong hold on the minds of biological investigators, and is even now widely accepted. It is, however, becoming more and more clearly apparent that this conception expresses only a part of the truth, and that Schwann went too far in denying the influence of the totality of the organism upon the local activities of the cells. It would of course be absurd to maintain that the whole can consist of more than the sum of its parts. Yet, as far as growth and development are con
cerned, it has now been clearly demonstrated that only in a limited sense can the cells be regarded as co-operating units. They are rather local centres of a formative power pervading the growing mass as a and the physiological autonomy of the individual cell falls into the background. It is true that the cells may acquire a high degree of physiological independence in the later stages of embryological development. The facts to be discussed in the eighth and ninth chapters will, however, show strong reason for the conclusion that this is a secondary result of development through which the cells become, as it were, emancipated in a greater or less degree, from the general control. Broadly viewed, therefore, the life of the multicellular organism is to be conceived as a whole ; and the apparently composite character, which it may exhibit, is owing to a secondary distribution of its energies among local centres of action? In this light the structural relations of tissue-cells becomes a question of great interest ; for we have here to seek the means by which the individual cell comes into relation with the totality of the organism, and by which the general equilibrium of the body is maintained. It must be confessed that the results of microscopical research have not thus far given a very certain answer to this question. Though the tissue-cells are often apparently separated from one another by a non-living intercellular substance, which may appear in the form of solid walls, it is by no means certain that their organic continuity is thus actually severed. Many cases are known in which division of the nucleus is not followed by division of the cell-body, so that multinuclear cells or .syncytia are thus formed, consisting of a continuous mass of protoplasm through which the nuclei are scattered. Heitzmann long since contended ('73), though on insufficient evidence, that division is incomplete in nearly all forms of tissue, and that even when cell-walls are formed they are traversed by strands of protoplasm by means of which the cell-bodies remain in organic continuity. The whole body was thus conceived by him as a syncytium, the cells being no more than nodal points in a general reticulum, and the body forming a continuous protoplasmic mass.