The simple method of propagation of the Moneron by self-division is, in reality, the most universal and most widely spread of all the different modes of propagation; for by the 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 (many Protista and single-celled plants), 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.[(39)] 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-substance or body (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 it 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 many-celled 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, Amœba sphærococcus, by self-division. A. The enclosed Amœba, 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 Amœba, 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 Amœbæ, as a rule propagate themselves in the simplest way 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 egg-cleavage (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.
Fig. 3.—Egg of a mammal (a simple cell).a. The small kernel speck or nucleolus (the so-calledgerm-spot of the egg). b. Kernel ornucleus (the so-called germ-bladder of the egg).c. Cell-substance or protoplasm (the so-calledyolk of the egg). d. Cell-capsule or membrane(membrane of the yolk) of the egg; called inmammals, on account of its transparency, Membranapellucida. | ||
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, Amœba (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 (Fig. 3, 4), develops out 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, the cell-kernel of the egg (the so-called germinal 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, thirty-two, etc., and finally there is produced a globular mass of very numerous little cells (Fig. 4 D). 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.