PROTOPLASM titles, which renders it sometimes extremely soft and nearly liquid, and sometimes hard and firm like leather ; but it is usually of a medium degree of density. Its more prominent physi- cal qualities are excitability and contractility, which Kuhne and others have made a special subject of investigation. On examining the numerous substances constituting the various organs of the higher animals with the micro- scope, it appears that they all consist of a large number of minute elements, known since Schleiden and Schwann (1838) by the name of cells ; and in these cells protoplasm is the old- est, most primordial, and most important con- stituent. In every real cell there is, besides protoplasm, and while still alive and indepen- dent, a second important constituent, the cel- lular germ, so called (nucleus or cytoblast) ; but even this germ consists of an albuminous chemical compound which is closely related to Srotoplasm, and was originally produced from
- by an exceedingly slight chemical alteration.
The germ is usually a smaller and firmer forma- tion within the protoplasm of the cell. Inas- much as the idea of an organic cell, as now adopted by histologists, rests on the presence of two different essential parts in this ele- mentary organism, the internal cell and the external protoplasm, wo must distinguish also two different kinds of elementary organisms : gormless cytods, as moners for example, and the real germ-enclosing cells, which originate from the former by secreting in the inte- rior of the small mass of protoplasm a true germ or nucleus. Cells of the simplest kind consist only of protoplasm with a nucleus, while in general the cells of animal or vege- table bodies have also other constituents, par- ticularly and frequently an enclosing skin or capsule (the cellular membrane), also crystals, grains of fat, pigments, and the like, within the protoplasm. But all of these parts came into being only secondarily through the chem- ical action of protoplasm; they are but the internal and external products of protoplasm. (Raeckel's Qenerelle Morphologic, vol. i., p. 279.) The single cell of the simplest kind is able to exist as an independent organism. Many of the lowest plants and animals, and also many neutral protista (which are nei- ther animals nor plants), retain for life the character of a simple cell. Such unicellular organisms of the simplest kinds are the anufba, found in large numbers as well in fresh as in salt water. Amoeba? are simple naked cells of various and varying forms. The whole differ- ence between them, especially protamceba, and certain moners, is that they have a germ. It is probable that this germ of the amoeba (as may be supposed to be the case with many and perhaps all other cells) is only an organ of propagation, and hence of heredity ; while all the other functions, alimentation, motion, and sensation, are performed by the proto- plasm. This seems to indicate that at the re- production of the cells, which is usually effect- ed by segmentation, it is the germ which first divides in two, and that the protoplasm after- ward gathers around each of the two sister germs till it also falls in two. It is impos- sible to distinguish from the common amoeba? the cellular ovules of many of the inferior ani- mals, as for example the sponges, medusa?, and other plant-like animals. "With these the eggs are simple naked cells, which, with the spon- ges especially, sometimes crawl about inde- pendently in the body of the animal, giving rise to the idea that they were a class of para- sitic amoeba?. But with other animals also, and with most plants, the eggs of which general- ly obtain subsequently special and often very complicated encasements and other additions, every egg is originally a simple cell. The semi- nal elements of the male are also only simple cells, and the entire mysterious process of fruc- tification is after all nothing but the fusion or concrescence of two different cells, the one a female egg cell, and the other a male semen cell. In consequence of this fusion the germs of the two combined cells dissolve, and therewith the young, newly generated individual begins his existence as a simple cytod, or a small gorm- less ball of protoplasm. But inside of this cytod soon arises a new germ, which turns it again into a cell, and this simple cell forms by oft repeated segmentation an accumulation of cells. Out of this heap are produced by secretion certain germinal layers or " germ leaves," and out of these proceed all the other organs of the complete being. Each of these organs again originally consists only of cells, and in all of these cells the essential constituent parts are only the germ and protoplasm : the germ as the elementary organ of propagation and heredity, protoplasm as the elementary organ of all the other functions, sensation, motion, alimentation, and adaptation. Cells and cytods, therefore, are true elementary or- ganisms, independent minute forms of life, which either in the lowest existences continue to live independently, or in the higher or- ganisms combine in numbers to form a com- munity. Cells and cytods are the veritable " formers " of life, or plastids. The most an- cient and primordial forms of plastids are cytods, the whole body of which consists of protoplasm, in which the germs are internally produced, and from which therefore the cells proceed. As a matter of course, to the infinite varieties presented by the organic forms and vital phenomena in the vegetable and animal kingdom, corresponds an equally infinite va- riety of chemical composition in the proto- plasm. The most minute homogeneous con- stituents of this "life substance," the proto- plasm molecules, or plastidules, as they are called by Elsberg, must in their chemical com- position present an infinite number of ex- tremely delicate gradations and variations. The atoms of carbon, hydrogen, nitrogen, oxy- gen, and sulphur, which compose each of the plastidnles, must enter into an infinite number
