The force of gravitation which is at work in crystal-formation in the simple chemical body exhibits just as definite a direction as that which appears in the plasm in cell-construction. In this and other respects the comparison of the cell with the crystal, which was made even by the founders of the cell-theory, Schleiden and Schwann, in 1838, is thoroughly justified, though it is not correct in some other aspects. When the crystal is formed in the mother-water, the homogeneous particles of the chemical substance arrange themselves in a perfectly definite direction and order, so that mathematical planes of symmetry and axes arise within, and definite angles at the surface. On the strength of this, modern crystallography distinguishes six different systems of crystals. But, in different conditions, the same substance may crystallize in two or even three different systems (dimorphism and trimorphism of the crystal); thus, for instance, carbonate of lime crystallizes as calcspar in the hexagonal, and as arragonite in the rhombic system. If Reinke would be consistent, he ought to postulate a "dominant" for every crystal, to control the order and direction of the particles in its formation. He makes the curious statement (in 1899) that direction "is not a measurable magnitude" like energy, and so is not subject, like it, to the law of substance. We can mathematically determine the direction of the constructive force in the crystal just as well as in the cell.
If we comprise under the head of cosmokinesis the whole of the movements of the heavenly bodies in space, we cannot deny that they have a definite direction in detail, although our knowledge of this is still very incomplete. We can calculate the distances and speeds and movements of the planets round the sun with mathematical accuracy; and we gather from our astronomical observations and calculations that a similar regularity prevails in the movements of the other countless bodies in infinite space. But we do not know either the first impulse to these complex movements or their final goal. We can only conclude from the great discoveries of modern physics, supported by spectrum analysis and celestial photography, that the universal law of substance on the one side and the law of evolution on the other control the gigantic movements of the heavenly bodies just as they do the living swarm of tiny organisms that have inhabited our little planet for millions of years. Reinke ought, consistently, to admire the cosmic intelligence of the Supreme Being in these movements of the cosmic masses and its emanations, the "dominants," in the actual direction of their movements, as much as he does in the plasma-flow in the tiny organism.
The manifold gradation of vital movement which we find everywhere in the higher organisms is not without expression even in the protist realm. In this respect the chromacea, the simplest forms of vegetal monera, and the bacteria, which we regard as corresponding animal forms, developed from the former by metasitism, are of great interest. As microscopic scrutiny fails to detect any purposive organization in these unnucleated cells, and it is impossible to discover different organs in their homogeneous plasma-body, we have to look upon their movements as direct effects of their chemical molecular structure. But the same must be said also of a number of nucleated cells, both among the protophyta and the protozoa; only in this case the structure is less simple, in so far as both the nucleus itself and the surrounding cell-body exhibit, in indirect division, complicated movements in the plasm (caryokinesis). Apart from these, however, there is nothing to be seen in many unicellular beings (e.g., paulotomea, or calcocytea) that we need call "vital movement." On the border between the organic and inorganic worlds we have, as regards movement, the simplest forms of the chromacea, chroococcacea. We can see no vital movement in these structureless particles of plasm except slight changes of form, which occur when they multiply by cleavage. The internal molecular movements of the living matter, which effect their simple plasmodomous metabolism and growth, lie beyond our vision. The reproduction itself, in its simplest form of self-cleavage, seems to be merely a redundant growth, exceeding the limit of individual size for the homogeneous plasma-globule (cf. chapters ix. and x.).
The great majority of the protists have the appearance of real, nucleated cells. Hence we have to distinguish two different forms of movement in the unicellular organism—the inner movement in the caryoplasm of the nucleus and the outer in the cytoplasm of the cell-body; the two enter into close mutual relations during the remarkable process of partial resolution of the nucleus (caryolysis). In this modification and partial dissolution of their constituents we observe, during indirect cell-division, certain complicated movements (the significance of which is as yet entirely unknown), that are accomplished by both the granules of chromatin and the threads of achromin, and which are comprised under the head of nuclear movements (caryokinesis). It has lately been attempted to explain them on purely physical principles. The same may be said of the internal flow of the plasm which we find in the plasmodia of the amœbæ and mycetozoa, and in the endoplasm of many of the protophyta and protozoa.
The slow displacement of the molecules of plasm which is at the bottom of these plasma-movements also causes a variety of external changes of form in simple naked cells. Variable processes like folds or fingers (the "fold-feet," lobopodia) appear on their surface. As they are best observed in the common amœbæ (naked nucleated cells of the simplest kind), they are called amœboid movements. With these is connected the variable movement of the larger rhizopods, the radiolaria and thalamophora, in which hundreds of fine threads radiate from the surface of the naked plasma-body. A number of recent experts on the rhizopods, such as Bütschli, Richard Hertwig, Rhumbler, and others, have attempted to trace to purely physical causes this varying formation of pseudopodia, and their branching and net-like structure (without definite direction).
It is more difficult to do this in the case of the most highly differentiated of the protozoa, the infusoria. With these the free movement of the unicellular protozoon is farther advanced through the formation of permanent hairlike processes (long single lashes in the flagellata, and a number of short lashes in the ciliata) on the cell-surface and the movement of these by contraction and expansion, like the limbs, tentacles, and bones of the higher animals. The apparent spontaneity and various modulation in the ever-changing movements of these cell-feet is, in many of the infusoria, so like the autonomous voluntary movements in the metazoa that several experts on the infusoria have been moved on this account to ascribe individual (and even conscious) souls to them. Hence the difference between the various kinds of living movement is already very considerable before we leave the kingdom of the protists. On the one hand, the lowest monera (chromacea) join on directly to inorganic phenomena. On the other hand, the highly differentiated infusoria (ciliata) show so great a resemblance to the higher animals in their differentiated and autonomous movements that they have been credited with the possession of "free-will." There is no such thing as a sharp division.
In a large section of the higher protozoa differentiated organs of movement are developed, which may be compared to the muscles of the metazoa. In the cytoplasm threadlike, contractile structures are formed, and these have, like the muscular fibres of the metazoa, the power to contract and expand again in definite directions. These myophæna or myonema form, in many of the infusoria, both ciliata and flagellata, a special thin layer of parallel or crossed fibres underneath the exoplasm or the hyaline skin-layer of the cell. The metabolic body of the infusorium may be altered in various ways by the autonomous contraction of these. Special instances of these myophæna are the myophrisca of the acantharia—contractile threads which surround the radial needles of these radiolaria like a crown. They are found in their outer gelatine envelope, the calymma, and by their contraction extend it, and so lessen the specific gravity.
Many of the aquatic protophyta and protozoa have the power of autonomous and independent locomotion, and this often has the appearance of being voluntary. Among the simplest fresh-water protozoa are the arcellina or thecolobosa (difflugia, arcella), little rhizopods that are distinguished from the naked amœbæ by the possession of a firm envelope. They usually creep about in the slime at the bottom, but in certain circumstances rise to the surface of the water. As Wilhelm Engelmann has shown, they accomplish this hydrostatic movement by means of a small vesicle of carbonic acid, which expands their unicellular body like an air-balloon; the specific weight of the cell-body, which is of itself heavier than water, is sufficiently lowered by this. The same method is followed by the pretty radiolaria which live floating (as plankton) at various depths of the sea. Their unicellular (originally globular) body is divided by a membrane into a firm inner central capsule and a soft outer gelatine covering. The latter, known as the calymma, is traversed by a number of water-vesicles or vacuoles. As a result of an osmotic process, carbonic acid may be secreted or pure water (without the salt of the sea-water) be imbibed in these vacuoles; by this means the specific gravity of the cell is lessened, and it rises to the surface. When it desires to make itself heavier and sink, the vacuoles discharge their lighter contents. These hydrostatic movements of the radiolaria (for which the myophrisca, still more complicated structures, have been developed in the acantharia) attain by simple means the same end that is accomplished in the siphonophora and fishes by air-filled and voluntarily contractile swimming-bladders.
Numbers of the unicellulars alter their position very characteristically by secreting a thick mucus at one side of their body and fastening this to the ground. If the secretion continues, a longish jelly-like stalk is produced by which the cell slowly pushes itself along, like a boat with a rowing-pole. This secretory locomotion is found, among the protophyta, in the desmidiacea and diatomes, and in some of the gregarinæ and rhizopods among the protozoa. The peculiar rolling movements of the oscillaria (threadlike chains of blueish-green unnucleated cells, closely related to the chromacea) are also effected by the secretion of mucus. On the other hand, it is probable that the sliding movements of many of the diatomes are due to fine processes (vibratory hairs?) in the plasm, which proceed either out of the seams (raphe) of the bivalvular silicious shells or through the fine pores in them.