It must be emphasized that living things are not homogeneous, but possess structural and physiological organization. Animals and plants are made up of various parts adapted for certain purposes. They exhibit 'a viable unity' and so stand in sharp contrast with objects comprising the inorganic world as, for instance, rocks and rivers. Accordingly animals and plants are referred to as organisms. Moreover, as we have seen, the organizational units of all living things are cells, and so it follows that cell structure is a direct or indirect expression of all the unique life characteristics that we are about to survey. A few of the details of cell structure are necessary for an appreciation of the organization of organisms.
It will be recalled that the protoplasm of all typical cells is differentiated into two chief parts: the cytoplasm, or general ground work which makes up the bulk of the cell; and the nucleus, a more or less clearly defined spherical body, situated near the center of the cytoplasmic mass.
Cytoplasm. The cytoplasm may be considered the less specialized protoplasm of the cell, and its appearance and other characteristics are those which have been outlined in our discussion of protoplasm. With that in mind, for the sake of definiteness, we may consider its basis as consisting of a meshwork, composed of innumerable, minute granules which permeate an apparently homogeneous ground-substance, or hyaloplasm. Distributed through out the cytoplasm are usually various lifeless inclusions such as granules of food, droplets of water or oil, vacuoles of cell sap, crystals, etc., representing materials which are to be, or have been, a part of the living complex, or are by-products of the vital processes. This passive material is frequently referred to as metaplasm, but it is quite evident that such a term stands for no essential morphological part of the cell, and we have no absolute criterion to distinguish between some granules which are regarded as metaplasmic in nature and others which are ordinarily considered active elements of the cytoplasm. But there are various undoubtedly active bodies besides the nucleus in the cytoplasm. Chief among these are the centrosome which plays an essential part in cell reproduction, and the plastids, mitochondria, and golgi bodies which apparently are the seat of various special physiological activities.
The cytoplasm, since it forms the general groundwork, is that part of the cell which comes most closely into relations with the environment, and accordingly near the surface it is frequently