. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. 4 CYTOLOGY chap. case of course the cytoplasm inside must form a new membrane round its periphery. Examples of cell membranes which have become relatively free from the cytoplasm by which they were secreted are the cellulose cell walls of most plant cells and the vitelline membranes of many eggs. (3) The Centrosome (see also p. 21) As we shall see, during di\dsion of the nucleus the centrosomes are the dynamical centres of the cell. Even when the nucleus is not actually in process of dividing, the centrosome may exert
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. 4 CYTOLOGY chap. case of course the cytoplasm inside must form a new membrane round its periphery. Examples of cell membranes which have become relatively free from the cytoplasm by which they were secreted are the cellulose cell walls of most plant cells and the vitelline membranes of many eggs. (3) The Centrosome (see also p. 21) As we shall see, during di\dsion of the nucleus the centrosomes are the dynamical centres of the cell. Even when the nucleus is not actually in process of dividing, the centrosome may exert a powerful influence on the topographical arrangements both of the nuclear constituents and of the cytoplasm. This is specially well illustrated in what is known as the "bouquet" stage in gameto- genesis (p. 33), where the chromosomes are in the form of U-shaped loops, so orient- ated that their free ends are directed towards the centro- some and the apices of the loops towards the opposite pole of the nucleus. In these cells the cytoplasm is heaped up round the centro- some, which is embedded in a mass of chondriosomes, " chromidia," etc., which it 191).. nm Fig. I. Diagram of a Cell, a, alveoli of cytoplasm ; c, chromatin in the form of fine granules (chromioles) embedded in the linin meshwork; ce, centrosome, .containing centriole ; cli, chondrio- some; mb, metaplastic body ; nm, nuclear membrane ; p, plasmosome. appears to have attracted round itself (p. (4) Chondriosomes These are minute granules or filaments embedded in the cytoplasm. They are discussed in Chapter VI. (5) Metaplastic Bodies are non-living material included in the cytoplasm, such as yolk granules, fat globules, excretory and secretory granules, etc. B. THE NUCLEUS The study of the nucleus is in many respects easier than that of the cytoplasm. There are several reasons for this, amongst which may be mentioned the comparative coarseness of its structure, and the strong, selective affinit
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