minous particles, and are there digested, being absorbed by simple diffusion (endosmosis).

Just as simple as the process of nutrition is the propagation of these primitive creatures, which in reality we can neither call animals nor plants. All Monera propagate themselves only in a non-sexual manner by monogony; and in the simplest case, by that kind of monogony which we place at the head of the different forms of propagation, that is, by self-division. When such a little globule, for example a Protamoeba or a Protogenes, has attained a certain size by the assimilation of foreign albuminous matter, it falls into two pieces; a pinching-in takes place, contracting the middle of the globule on all sides, and finally leads to the separation of the two halves (compare Fig. 1). Each half

FIG. 1.-Propagation of the simplest organism, a Moneron, by self-division. A. The entire Moneron, a Protamoba. B. It falls into two halves by a contraction in the middle. C. Each of the two halves has separated from the other, and now represents an independent individual.

then becomes rounded off, and now appears as an independent individual, which commences anew the simple course of the vital phenomena of nutrition and propagation. By the separated half becoming gradually replaced by growth, this regeneration destroys a part for the good of the whole. In other Monera (Vampyrella and Glodium), the body in

the process of propagation does not fall into two, but into four equal pieces, and in others, again (Protomonas, Protomyxa, Myxastrum), at once into a large number of small globules of mucus, each of which again, by simple growth, becomes like the parent body (Plate I.). Here it is evident that the process of propagation is nothing but a growth of the organism beyond its own individual limit of size.

The simple method of propagation of the Moneron by selfdivision is, in reality, the most universal and most widely spread of all the different modes of propagation; for by this same simple process of division, cells also propagate themselves. Cells are those simple organic individuals, a large number of which constitute the bodies of most organisms, the human body not excepted. With the exception of the organisms of the lowest order, which have not even the perfect form of a cell (Monera), or during life only represent a single cell (like many Protista), the body of every organic individual is composed of a great number of cells. Every organic cell is to a certain degree an independent organism, a so-called "elementary organism," or an "individual of the first order." Every higher organism is, in a measure, a society or a state of such variously shaped elementary individuals, variously developed by division of labour. Originally every organic cell is only a single globule of mucus, like a Moneron, but differing from it in the fact that the homogeneous albuminous substance has separated itself into two different parts, a firmer albuminous body, the cell-kernel (nucleus), and an external, softer albuminous body, the cell-slime (protoplasma). Besides this, many cells later on form a third (frequently absent)

distinct part, inasmuch as they cover themselves with a capsule, by exuding an outer pellicle or cell-membrane (membrana). All other forms of cells, besides these, are of subordinate importance, and are of no further interest to us here.

Every organism composed of many cells was originally a single cell, and becomes many-celled owing to the fact that the original cell propagates itself by self-division, and that the new individual cells originating in this manner remain together, and by division of labour form a community or a state. The forms and vital phenomena of all manycelled organisms are merely the effect or the expression of all the forms and vital phenomena of all the individual cells of which they are composed. The egg, from which most animals and plants are developed, is a simple cell.

FIG. 2.-Propagation of a single-celled organism, Amoeba sphærococcus, by self-division. A. The enclosed Amoeba, a simple globular cell consisting of a lump of protoplasm (c), which contains a kernel (b) and a kernel speck (a), and is surrounded by a cell-membrane or capsule. B. The free Amoeba, which has burst and left the cyst or cell-membrane. C. It begins to divide by its kernel forming two kernels, and by the cell-substance between the two becoming contracted. D. The division is completed by the cell-substance likewise falling into two halves (Da and Db).

The single-celled organisms, that is, those which during life retain the form of a single cell, for example, the Amoebæ (Fig. 2), as a rule propagate themselves in the simplest way

VOL. I.

by self-division. This process differs from the previously described self-division of the Moneron only in the fact that at the commencement the firmer cell-kernel (nucleus) falls into two halves, by a pinching-in at its middle. The two young kernels separate from each other and act now as two distinct centres of attraction upon the surrounding softer albuminous matter, that is, the cell-substance (protoplasma). By this process finally the latter also divides into two halves, and there now exist two new cells, which are like the mother cell. If the cell was surrounded by a membrane, this either does not divide at all, as in the case of eggcleavage (Fig. 3, 4), or it passively follows the active pinching-in of the protoplasm; or, lastly, every new cell exudes a new membrane for itself.

The non-independent cells which remain united in communities or states, and thus constitute the body of higher organisms, are propagated in the same manner as are independent single-celled organisms, for example, Amoeba (Fig. 2). Just as in that case, the cell with which most animals and plants commence their individual existence, namely, the egg, multiplies itself by simple division. When an animal, for instance, a mammal (Figs. 3, 4), develops out

FIG. 3.-Egg of a mammal (a simple cell). a. The small kernel speck or nucleolus (the socalled germ-spot of the egg). b. Kernel or nucleus (the so-called germ-bladder of the egg). c. Cell-substance or protoplasm (the so-called yolk of the egg). d. Cell-capsule or membrane (membrane of the yolk) of the egg; called in mammals, on account of its transparency, Mem. brana pellucida.

of an egg, this process of development always begins by the simple egg-cell (Fig. 3) forming an accumulation of cells

(Fig. 4) by continued self-division. The outer covering, or cell-membrane, of the globular egg remains undivided. First,

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FIG. 4.-First commencement of the development of a mammal's egg, the so-called "cleavage of the egg" (propagation of the egg-cell by repeated self-division). A. The egg, by the formation of the first furrow, falls into two cells. B. These separate by division into four cells. C. The latter have divided into eight cells. D. By repeated division a globular accumulation of numerous cells has arisen.

the cell-kernel of the egg (the so-called germical vesicle) divides itself into two kernels, then follows the cell-substance (the yolk of the egg) (Fig. 4 A). In like manner, the two cells, by continued self-division, separate into four (Fig. 4 B), these into eight (Fig. 4 C), into sixteen, thirtytwo, etc., and finally there is produced a globular mass of very numerous little cells (Fig. 4D). These now, by further increase and heterogeneous development (division of labour), gradually build up the compound many-celled organism. Every one of us, at the commencement of our individual development, has undergone the very same process as that represented in Fig. 4. The egg of a mammal-represented in Fig. 3, and its development in Fig. 4-might as well be that of a man, as of an ape, dog, horse, or any other placental mammal. Now, when we examine this simplest form of propagation, this self-division, it surely cannot be considered wonderful that the products of the division of the original organism should possess the same qualities as the parental