II.
The way in which protoplasm gets its chemical requisites for growth is doubtless simply by absorbing them. Some of the lower structureless forms carry this to an absurd extreme, for when two individuals meet they fuse, and each no doubt claims to have eaten the other. As, moreover, the first thing which a cell does when it grows is to divide, the whole proceeding looks rather futile. But ready-made protoplasm of an assimilable shape is rare, and it is not often that a cell, unless it be a plant or a parasite, finds itself in a substance which can be handed straight to the nucleus without further elaboration. Usually the cell has to discharge from itself a reagent, which will develop the right chemical qualities in the matter it wants to absorb. This substance is known as an enzyme, or ferment. Ferments, however, are an expense to the cell, requiring a certain effort for their production; so, in order that they may be economized, they are, in the higher forms, poured over the food while it is in an enclosed cavity, or stomach. In the simplest animals, consisting of a single cell, the protoplasm simply flows round the particle of food, and it is ‘ingested’ with a drop of water. Into this ‘food vacuole’ the ferments are secreted, and when all that is useful has been dissolved out and absorbed, the bubble moves to the surface and bursts; or, to put it differently, the cell flows on its way, and the vacuole, with any shell or refuse it may contain, gets left behind. ([See Diagram 1.]) In other cells which are constant in shape there is an opening leading to the interior of the cell. Round this there are little projecting threads, which beat the water regularly. In some positions these threads enable the cell to swim, but here their duty is to cause a current and wash particles of food down the primitive throat into the interior, where, as in the preceding case, they become enclosed in a vacuole. ([See Diagram 2.])
Diagram 1.—The Amœba.
Diagram 2.—Paramœcium.
Moving a stage higher, we find animals consisting of several cells. Of these it is only natural to suppose that some have greater enzyme-forming powers than others.
Diagram 3.—Development of an Embryo: First Stage.
Diagram 4.—Formation of a Digestive Cavity.
Diagram 5.—Cross Section of a Developing Embryo.
A step higher in the animal scale, or a further advance in the development of the schematic embryo (depicted in [Diagrams 3 to 6]), and we find that these special digestive cells are losing their sturdier qualities and being placed in a position protected by cells which have specialized in another direction. This is shown in [Diagram 4], where the hollow ball of cells which resulted from the repeated division of one cell is represented in section. One side of the ball is pushed in, and now the beast consists of two layers of cells, an outer protecting and an inner digesting (Hydra and sea-anemone). Soon, however, it is found more convenient to have a tube for digesting food, for then different substances can be digested and absorbed in different parts; and the refuse, of which the animal can make no use, need not be brought back to the mouth to be got rid of.
This, however, requires a number of other changes in the structure of the animal, which are roughly shown in [Diagrams 5 and 6]. It is not to our purpose here to discuss the development of animals or an animal; but the figures are worth glancing at, as they show not only how certain of the cells are set apart for digesting food, but also that a large body consists really only of a mass of protoplasm, composing kindred cells of common origin.
Diagram 6.—Showing Development of an Embryo.
Now, for obvious reasons, the longer, within certain limits, this tube is the better. All sorts of different food-stuffs have to be acted upon in it, and some offer considerable resistance to digestion; and the further they have to travel in the tube, the more chance there is of their being successfully treated. Besides, different parts have different functions, and the longer the tube—again within necessary limits—the greater scope is there for division of labour, and consequent economy. The comparative length of the alimentary canal is not the same in all animals by any means. Carnivorous animals, like the cat, whose food is soft and easily digested, have a fairly short one. Herbivora, like the sheep, whose food is difficult to digest and mixed with much husk, which is wholly indigestible, have a comparatively very long one. Man, who is omnivorous, but eats less and more judiciously chosen food than either of the above classes, has one of medium length. But in all cases among the higher animals there is an attempt made to obviate the necessity of increasing the length of the animal by coiling the tube within the body. The annexed diagram ([7]) illustrates this principle. It shows a schematic animal whose digestive canal is much longer than itself.
Diagram 7.—Showing how the Digestive Canal is Lengthened.
Diagram 8.—Cross-section of the Digestive Tube.
The digestive canal has, however, another function. The cells which compose it have not only to secrete juices, to convert the food into a usable form; they have then to absorb it. The nearer a particle of food is to the wall of cells, the sooner it is reached by these juices, and the less chance there is of useful material being swept away and lost. In view of this fact, along certain tracts the digestive canal is folded inwards, and there are projections, which increase the number of cells to secrete and their opportunities of absorption. ([See Diagram 8.])
Diagram 9.—Showing how Glands arise.
Here again we have an illustration of a constantly recurring need, with a device for meeting it—increase of surface without increase of bulk. We met with it before in the cellular system; we shall meet with it again in glands, lungs, and brain, at least. The importance of a device for gaining this end is apparent when one remembers what the comparative value of surface and bulk is to an animal, and that, while surface increases by the square, bulk increases by the cube.
The principle is pressed to an extreme, together with the allied principle of division of labour, in glands. The object of these is to increase the number of secreting cells, and, as they are delicate, to keep them protected from contact with coarse particles of food. And, in order that nothing may interfere with their efficiency, they are absolved from the duty of absorbing. Hence tubes grow out from the cavity of the alimentary canal lined with the same cells, but, as no food ever enters, the cells which line them devote themselves entirely to pouring out digestive juices. Glands differ considerably in structure and in the liquids which they secrete. Some are very small; some, like the liver, very large. In some the tube is very short, in some long, coiled and branched, and sometimes the gland is connected with the surface by more or less of a duct. Some glands only secrete one enzyme, some several. In each, however, the principle is that shown in [Diagram 9], no matter how its structure is masked by the bloodvessels and supporting cells or connective tissue which envelop it.
After a meal, or, rather, when the process of digestion is over and the animal is beginning to think about its next, the gland cells start preparing their enzyme. There is great activity in the nucleus, and granules stream out from it towards the lumen of the gland in much the same way, to take a homely illustration, as bubbles in some effervescing drink form at the bottom of the tumbler and rise till the surface is covered with foam. At the right moment these granules are discharged, just as the bubbles on the surface of a liquid break at a slight jog. They are usually not the ferment or enzyme, but its precursor, a substance which only turns into the ferment when it gets outside the cells. The ferments, when formed, are very peculiar substances about which we should like the chemist to tell us more, though great advances have been made in our knowledge of them lately.
Among other peculiarities, one may mention that, though they will keep indefinitely if bottled, they are easily destroyed by too extreme a temperature or too acid or alkaline surroundings, that their composition is entirely unknown, and, strangest of all, that they do not become used up. A given amount of rennet will clot any amount of milk within reasonable limits, and yet remain rennet. The clergyman has been quoted as an illustration of the action of a ferment, and he makes a good one. He can make any number of suitable men and women into married couples, and yet his own state is unchanged.