The protoplasm of all typical cells is differentiated into two parts: the Cytoplasm, or general groundwork which makes up the bulk of the cell; and the Nucleus, a restricted, clearly defined area, usually situated near the center of the cytoplasmic mass.
The cytoplasm may be considered the more generalized 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 mesh work, com posed of innumerable, minute granules which permeate an apparently homogeneous hyaline ground-substance. Dis tributed throughout 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. Specialized living cytoplasmic bodies, known as PLASTIDS, are sometimes also present. Finally, within the cytoplasm in the vicinity of the nucleus, there is frequently visible a differentiated area containing a Centrosome, an important cell organ which is especially active during cell reproduction.
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 modified somewhat in texture and consistency so that an outer region, or ECTOPLASM, may be distinguished from an inner, or Endoplasm. Again, the cell may form about itself a definite membrane or a heavy cell wall. Nearly all gradations exist between highly differentiated cytoplasm (ectoplasm) and definite membranes and cell walls. The ectoplasm is certainly a part of the living protoplasm, while certain types of membranes and cell walls must be regarded as non-living, though in many cases they are direct trans formations of the living materials which grow and play an important, indeed a crucial, part in controlling directly or indirectly the flow of matter and energy to and from the cell and its surroundings.