[D] It will be well here to introduce the technical terms for these changes. The general term for chemical actions occurring in the tissues of a living creature is _metabolism_; where the change is of such a nature that complex and unstable compounds are built up and stored for a while, it is called _anabolism_; where complex unstable compounds break up into less complex and relatively stable compounds, the term _katabolism_ is applied. We shall speak of anabolic changes as _constructive_; katabolic, as _disruptive_, or sometimes, _explosive_. [E] I do not mean, of course, to imply that there is no reconstruction during activity, but that it is then distinctly outbalanced by disruptive changes. CHAPTER III. REPRODUCTION AND DEVELOPMENT. We have now to turn to a fresh aspect of animal life, that of reproduction; and it will be well to connect this process as closely as possible with the process of life in general, of which it is a direct outcome. It will be remembered that, in the last chapter, it was shown that the essential feature in the process of life is the absorption by living protoplasm of oxygen on the one hand and nutritive matter on the other hand, and the kneading of these together, in subtle metabolism, into unstable compounds, which we likened to explosives. This is the first, or constructive, stage of the life-process. Thereupon follows the second, or disruptive, stage. The unstable compounds break down into more stable products,--they explode, according to our analogy; and accompanying the explosions are manifestations of motor activity--of heat, sometimes of light and electrical phenomena. But in the economy of nature the products of explosion are often utilized, and in the division of labour among cells the explosions of some of them are directed specially to the production of substances which shall be of permanent or temporary use--for digestion, as in the products of the salivary, gastric, and intestinal glands; for support, as in bone, cartilage, and skeletal tissue generally; or as a store of nutriment, in fat or yolk. The constructive products of protoplasmic activity seem for the most part to be lodged in the spaces between the network of formative protoplasm. The disruptive products--those of them, that is to say, which are of temporary or permanent value to the organism--accumulate either within the cell, sometimes at one pole, sometimes at the centre, as in the case of the yolk of eggs, or around the cell, as in the case of cartilage or bone.