The living substance, protoplasm,[6] has been termed a "jelly," a word, however, that is quite inapplicable to it in its living state. It is viscid, almost semi-fluid, and may well be compared to very soft dough which has already begun to rise. It resembles it in often having a number of spaces, small or large, filled with liquid (not gas). These are termed "vacuoles" or "alveoles," according to their greater or their lesser dimensions. In some cases a vacuole is traversed by strands of plasmic substance, just as we may find such strands stretching across the larger spaces of a very light loaf; but of course in the living cell these are constantly undergoing changes. If we "fix" a cell (_i.e._ kill it by {6}sudden heat or certain chemical coagulants),[7] and examine it under the microscope, the intermediate substance between the vacuoles that we have already seen in life is again found either to be finely honeycombed or else resolved into a network like that of a sponge. The former structure is called a "foam" or "alveolar" structure, the latter a "reticulate" structure. The alveoles are about 1 µ in diameter, and spheroidal or polygonal by mutual contact, elongated, however, radially to any free surface, whether it be that of the cell itself or that of a larger alveole or vacuole. The inner layer of protoplasm ("endoplasm," "endosarc") contains also granules of various nature, reserve matters of various kinds, oil-globules, and particles of mineral matter[8] which are waste products, and are called "excretory." In fixed specimens these granules are seen to occupy the nodes of the network or of the alveoli, that is, the points where two or three boundaries meet.[9] The outermost layer ("ectoplasm" or "ectosarc") appears in the live Amoeba structureless and hyaline, even under conditions the most favourable for observation. The refractive index of protoplasm, when living, is always well under 1.4, that of the fixed and dehydrated substance is slightly over 1.6. Again, within the outer protoplasm is found a body of slightly higher refractivity and of definite outline, termed the "nucleus" (Figs. 1, 2). This has a definite "wall" of plasmic nature, and a substance so closely resembling the outer protoplasm in character, that we call it the "nucleoplasm" (also "linin"), distinguishing the outer plasm as "cytoplasm"; the term "protoplasm" including both. Within the nucleoplasm are granules of a substance that stains well with the commoner dyes, especially the "basic" ones, and which has hence been called "chromatin." The linin is {7}usually arranged in a distinct network, confluent into a "parietal layer" within the nuclear wall; the meshes traversing a cavity full of liquid, the nuclear sap, and containing in their course the granules; while in the cavity are usually found one or two droplets of a denser substance termed "nucleoles." These differ slightly in composition from the chromatin granules[10] (see p. 24 f.).