STRUCTURE AND ACTIVITIES OF CELLS 



the endoplasmic reticulum together with the associated small granular 

 components. 



Another structure is almost universally found in the cytosome of the animal 

 cell. It is the cell center, a small area appearing homogeneous when seen 

 with the light microscope except for the presence of two small rods, the 

 centrioles; these characteristically lie at right angles to one another 

 (Fig. 2.6/1). Electron micrographs reveal an amazing uniformity in the 

 internal structure of these minute rods in all types of cells (Fig. 2.6D). 

 The cell center is actively involved in the process of nuclear division and will 

 be considered further in that connection (p. 39). In the region of the cyto- 

 some about the center in some but not all kinds of cells, it is possible to 

 demonstrate with special histological techniques an irregular, coarse net-like 

 structure known as the Golgi apparatus (Fig. 2.6/1). Electron micrographs 

 reveal in many cells a portion of the endoplasmic reticulum which lacks the 

 small granular components typically found on its membrane. It is called the 

 agranular reticulum and is associated with numerous vesicles; together, these 

 are assumed to be the Golgi apparatus (Fig. 2.6C). Little is known of the 

 biochemistry or function of this structure. 



We come now to the nucleus, which is usually rounded and located some- 

 what centrally in the cell (Fig. 2.6.4). The nucleus is surrounded by a distinct 

 nuclear membrane or envelope, as seen in the light microscope. Electron micro- 

 graphs reveal that the nuclear envelope actually consists of two closely associ- 

 ated layers, each of which is porous (Fig. 2.6F). Through the pores, which 

 are common to both layers, the ground substance of the nucleus is continuous 

 with the endoplasm. The space between the two layers of the nuclear 

 envelope is continuous with the canals of the endoplasmic reticulum by way of 

 pores in the outer layer. 



Nuclei examined with the light microscope after fixation and staining 

 appear to contain irregular, densely stained clumps of chromatin and one or 

 more rounded nucleoli disposed on a delicate network (Fig. 2.7.4). Nuclei 

 of living cells examined with the phase-contrast microscope appear empty 

 except for one or more nucleoli (Fig. 2.1B). Electron micrographs sub- 

 stantiate the inferences made from observations with phase-contrast lenses. 

 The nucleus is seen to contain numerous small granules distributed without 

 pattern (Fig. 2.6F); these are similar to the small granules of the cytosome. 

 The nucleolus is a densely packed mass of granules, sometimes in linear order, 

 not bounded by a membrane. Nucleoli are known to contain RNA and 

 protein. As we shall learn when nuclear division is described under the 

 discussion of cell division, very precisely arranged strands of chemical 

 molecules, the genonemata, or gene strings, exist in the nucleus. Their 

 dimensions are such that even the electron microscope cannot reveal them in 

 the intervals between divisions of the cell. It has been determined, however, 

 that genonemata are made up of desoxyribonucleic acid (DNA) which together 

 with characteristic proteins forms the chromosomes seen in the nucleus of 

 the dividing cell (pp. 39 and 40). The compounds of the genonemata are 



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