All organisms consist either--as do the simplest, mostly microscopic, plants and animals--of a single minute mass of protoplasm, or of a few, or of many, or of an enormous aggregation of such before-mentioned particles, each of which is one of those bodies named a "cell" (Fig. 3). Cells may, or may not, be enclosed in an investing coat or "cell-wall." Every cell generally contains within it a denser, normally spheroidal, body known as the nucleus. Now protoplasm is a very unstable substance--as we have seen many substances are whereof nitrogen is a component part--and it possesses active properties which are not present in the non-living, or inorganic world. In the latter, differences of temperature will produce motion in the shape of "currents," as we have seen with respect to masses of air and water. But in a portion of protoplasm, an internal circulation of currents in definite lines will establish itself from other causes. Inorganic bodies, as we have seen, will expand with heat, as they may also do from imbibing moisture; but living protoplasm has an apparently spontaneous power of contraction and expansion under certain external conditions which do not occasion such movements in inorganic matter. [Illustration: FIG. 3. CELL FROM A SALAMANDER. _n_, nucleus; _n'_, nucleolus embedded in the network of chromatin threads; _k_, network of the cell external to the nucleus; _a_, attraction-sphere or archoplasm containing minute bodies called centrosomes; _cl_, membrane enclosing the cell externally, _nl_, membrane surrounding the nucleus; _c_, centrosomes.] Under favoring conditions, protoplasm has a power of performing chemical changes, which result in producing heat far more gently and continuously than it is produced by the combustion of inorganic bodies. Thus it is that the heat is produced which makes its presence evident to us in what we call "warm-blooded animals," the most warm-blooded of all being birds. Protoplasm has also the wonderful power of transforming certain adjacent substances into material like itself--into its own substance--and so, in a sense, creating a new material. Thus it is that organisms have the power to nourish themselves and grow. An animal would vainly swallow the most nourishing food if the ultimate, protoplasmic particles of its body had not this power of "transforming" suitable substances brought near them in ways to be hereinafter noticed. Without that, no organism could ever "grow." The growth of organisms is utterly different from the increase in size of inorganic bodies. Crystals, as we have seen, grow merely by external increment; but organisms grow by an increment which takes place in the very innermost substance of the tissues which compose their bodies, and the innermost substance of the cells which compose such tissues; this peculiar form of growth is termed _intussusception_.