Section 55. Throughout life, millions of these cells retain their primary characters, and constitute the white corpuscles of blood, "phagocytes," and connective tissue corpuscles; others again, engage in the formation of material round themselves, and lie, in such cases, as gristle and bone, embedded in the substance they have formed; others again, undergo great changes in form and internal structure, and become permanently modified into, for instance, nerve fibres and muscle substance. The various substances arising in this way through the activity of cells are called tissues, the building materials of that complex thing, the animal body. Since such a creature as the rabbit is formed through the co-operation of a vast multitude of cells, it is called multicellular; the amoeba, on the other hand, is unicellular. The rabbit may be thus regarded as a vast community of amoeboid creatures and their products. Section 56. Figure IV., Sheet 3 represents, diagrammatically, embryonic tissue, of which, to begin with, the whole animal consists. The cells are all living, capable of dividing and similar, but as development proceeds, they differentiate, some take on one kind of duty (function), and some another, like boys taking to different trades on leaving school, and wide differences in structure and interdependence become apparent. Section 57. It is convenient to divide tissues into three classes, though the divisions are by no means clearly marked, nor have they any scientific value. The first of these comprises tissues composed wholly, or with the exception of an almost imperceptible cementing substance, of cells; the second division includes the skeletal tissues, the tissue of mesentery, and the connective and basement tissue of most of the organs, tissues which, generally speaking, consist of a matrix or embedding substance, formed by the cells and outside of them, as well as the cells themselves; and, thirdly, muscular and nervous tissue. We shall study the former two in this chapter, and defer the third division until later.