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.) [Illustration: DIAGRAM 1.—THE AMŒBA.] [Illustration: 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. [Illustration: DIAGRAM 3.—DEVELOPMENT OF AN EMBRYO: FIRST STAGE.] [Illustration: DIAGRAM 4.—FORMATION OF A DIGESTIVE CAVITY.] [Illustration: DIAGRAM 5.—CROSS SECTION OF A DEVELOPING EMBRYO.]