THE ORGANIZATION OF THE INDIVIDUAL 17 



above it, is a small and sometimes optically distinct body called the 

 centrosomc, containing a pair of very minute centrioles. 



All of the protoplasm outside of the nuclear membrane constitutes the 

 cytoplasm. Its surface is differentiated into a living cell membrane, capable 

 of regulating much of the cell's interchange of substances with the sur- 

 rounding medium or with other cells. Internally the cytoplasm may show 

 various specialized regions, structures, and inclusions. Among these are 

 thread-shaped structures known as mitochondria, which appear to be the 

 seat of important metabolic enzymes (organic catalysts), and Golgi 

 bodies, which may usually be demonstrated in animal cells but are of 

 unknown function. Other cytoplasmic structures which may or may not 

 be found in a given kind of cell include vacuoles and plastids. A vacuole 

 is a usually spherical space that contains fluid or suspended wastes or 

 cell products and that is enclosed in a cytoplasmic membrane. Plastids 

 are especially characteristic of plant cells; they are definitely organized 

 bodies of protoplasm surrounded by bounding membranes, and may be 

 spherical, ovoid, or sometimes ribbon-shaped or collarlike. They may be 

 colorless but often contain the characteristic plant pigments which give 

 color to leaves and flowers. In addition, cytoplasm will usually be found 

 to contain a changing variety of inclusions, granules or droplets of se- 

 creted substances, and of foods and wastes that are accumulated or 

 produced by the metabolic activities of the cell. 



The cell that we have briefly described is one in the so-called "resting" 

 stage, or condition. Actually such a cell is engaged in a host of metabolic 

 activities and is only "resting" in the sense that it is not in the process of 

 division. The discernible structures and activities of the resting stage, 

 however, give but little indication of the complex structure of the nucleus 

 or of the precise organization of the chromatin material into bodies called 

 chromosomes, which maintain their identities throughout all the nuclear 

 transformations that occur in cell division. 



MITOSIS 



The typical and almost invariable method of division of cells is a com- 

 plex process named mitosis (Greek, mitos, "thread") in allusion to the 

 conspicuous threadlike structures that appear in its early stages. Once 

 begun, the process is a more or less continuous one that lasts from 2 to 24 

 hours in different kinds of cells. But it is customary and convenient to 

 recognize four successive stages in the process: prophase, metaphase, 

 anaphase, and telophase. The events that take place during these stages 

 are somewhat diagrammatically illustrated in Fig. 2.2, which begins with 

 the resting stage of a cell containing six chromosomes. The following 

 account will be more easily understood if frequently compared with the 

 appropriate drawings. 



