From the foregoing description of the manner in which protoplasm responds to external stimuli, it may be gathered that it is capable of appreciating impressions from without; indeed, it can be said to feel. We can only judge of the sensitiveness of any creature by the manner in which it responds to stimuli, and we may therefore conclude that the smallest particle of living protoplasm is endowed with definite sensitiveness; this must be noted as one of the most striking properties of protoplasm.
Every particle of living protoplasm has the power of assimilation. Taking into its structure any nutrient matters it meets with, by flowing around them in the way mentioned, it brings them into direct contact with different parts of its protoplasmic substance. This nutrition of the cells gives rise to their growth, and finally leads to their reproduction. These facts will be more closely examined when speaking of their relation to cell life.
Fig. 36. Cells of the yeast plant in process of budding, between which are some bacteria.
Fig. 37. Cartilage from young animal showing the division of the cells (a, b, c, d).
When a certain size has been attained, the cell does not further increase, but prepares to bring forth a cell unit similar to itself. This is spoken of as the reproduction of cells.
Different kinds of cell reproduction have been observed, which are all, however, modifications of the same general plan. The first is that by the formation of a bud from the side of the parent cell; this bud increases in size, and finally detaches itself from the parent and becomes a separate individual. This process, which is called gemmation, can readily be seen in all its stages in growing yeast, where the torula cells have various-sized buds growing from them. If the newly-formed portion be large, nearly equal in size to the cell itself, the process receives the name of fission, or division. In well-marked typical fission the parent cell divides into two parts of equal size, each of which becomes a perfect individual. Various gradations may be traced between the two processes, so that it is difficult to draw any very distinct line between budding and fission. The budding and fission may be multiple; many buds and several units, products of division, may remain together, and form what is called a colony. When this multiple budding or division takes place, so that the new units are included within the body of the parent cell, then the process is called endogenous reproduction or spore formation. Just as there are gradations between budding and fission, so it is difficult to draw a hard and fast line between what may be called multiple fission and spore formation.
In tracing the stages of development of the highly differentiated cells of some tissues, we have to pass through a series of changes which form a cycle that may well be called the lifetime of the cell. The duration of this cycle varies greatly in different individual cells. Some cells are very short-lived, being destroyed by their act of secretion; others probably endure for the lifetime of the animal. The life history of all cells begins with the stage when they are composed entirely of indifferent protoplasm, in which various modifications are subsequently produced.
Let us take, as an example, a cell of the outer skin or cuticle, and examine its life history. The cuticle is made up of numerous layers of epithelial cells laid one on the other, and the surface cells are constantly being rubbed or worn off. These cells have their origin from the cells of the deepest layer, which is next to the supply of nutriment. This layer is made up of soft protoplasmic units, which have, no doubt, certain specific inherited characteristics, but apparently the same as the motile, sentient, growing protoplasm of an indifferent cell. By a process of fission or budding, constantly going on in this deepest layer of cells, new protoplasmic units are produced. These become distinct individuals, and occupy the position of the parent cell, which, having produced offspring, is moved one place nearer the surface, away from the supply of food. The new cell in time gives rise to offspring, and having attained reproductive maturity, in turn is moved onward to the surface. The result of this-is that its supply of nutrition diminishes, the evidences of reproductive activity disappear, and at a certain point all signs of protoplasmic life are lost. But on its way from the seat of its origin to the surface, it makes use of its limited supply of nutrition for the purpose of manufacturing a special kind of material which, if present at all, only occurs in the minutest traces in ordinary protoplasm. As the cell moves toward the surface, it loses its protoplasmic characters, becomes tougher and drier, and finally nothing but the special horny material remains. Thus, from the birth of the cell, its energies are devoted, first, to its own growth, then to the reproduction of its like, and finally to the formation of a material fitted to act as a mechanical protection to the surface of the skin. Having manufactured a certain amount of this material, the protoplasm dwindles, and finally disappears, so that the cell may be said to die. Its horny, insoluble and impermeable skeleton has, however, yet to do service in the outer layer of the skin while it is passing toward the surface, to be in its turn rubbed off.
Fig. 38. Cells of a fungus (Glaeocapsa) showing different stages (1-4) of endogenous division. (After Sachs).
Fig. 39. Division of Egg cell. (Gegenbatter).