For new material can only be absorbed by its surface. But as the cell grows, the proportion borne by its surface to its mass, continually decreases; therefore this surface must soon be too small to take in nourishment enough, and the particle, or cell, must therefore either die or divide. By dividing, its parts can continue the nutritive process till their surface, in turn, becomes insufficient, when they must divide again, and so on. Thus the term "feeding" has two senses. "To feed a horse," ordinarily means to give it a certain quantity of hay, oats or what not; and such indeed is one kind of feeding. But obviously, if the nourishment so taken could not get from the stomach and intestines into the ultimate particles and cells of the horse's body, the horse could not be nourished, and still less could it grow. It is this latter process, called assimilation, which is the real and essential process of feeding, to which the process ordinarily so called is but introductory.
Protoplasm has also the power of forming and ejecting from its own substance, other substances which it has made, but which are of a different nature to its own. This function, as before said, is termed secretion; and we know the liver secretes bile, and that the cow's udder secretes milk.
Here again we have an external and an internal process. The milk is drawn forth from a receptacle, the udder, into which it finds its way, and so, in a superficial sense, it may be called an organ of secretion. Nevertheless the true internal secretion takes place in the innermost substance of the cells or particles of protoplasm, of the milk-land, which particles really form that liquid.
FIG. 4. AMOEBA SHOWN IN TWO OF THE MANY IRREGULAR SHAPES IT ASSUMES. (After Howes.)
The clear space within it is a contractile vesicle. The dark body is the nucleus. In the right-hand figure there is shown a particle of food, passing through the external surface.
But every living creature consists at first entirely of a particle of protoplasm. Therefore every other kind of substance which may be found in every kind of plant or animal, must have been formed through it, and be, in fact, a secretion from protoplasm. Such is the rosy cheek of an apple, or of a maiden, the luscious juice of the peach, the produce of the castor-oil plant, the baleen that lines the whale's enormous jaws, as well as that softest product, the fur of the chinchilla. Indeed, every particle of protoplasm requires, in order that it may live, a continuous process of exchange. It needs to be continuously first built up by food, and then broken down by discharging what is no longer needful for its healthy existence. Thus the life of every organism is a life of almost incessant change, not only in its being as a whole, but in that of all its protoplasmic particles also.
Prominent among such processes is that of an interchange of gases between the living being and its environment. This process consists in an absorption of oxygen and a giving-out of carbonic acid, which exchange is termed respiration.
Lastly, protoplasm has a power of motion when appropriately acted on. It will then contract or expand its shape by alternate protrusions and retractions of parts of its substance. These movements are termed amoebiform, because they quite resemble the movements of a small animalcule which is named amoeba. (See Fig. 4.)