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 division 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 



nm \ *** r^ ~^/ s 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- 



Diagram of a Cell, a, alveoli of cytoplasm ; c, chromatin , . . . -L j j j 



in the form of fine granules (chromioles) embedded in the linin SOme, Which IS embedded in 

 meshwork; ce, centrosome, containing centriole ; ch, chondrio- ,. , , . 



some ; mb, metaplastic body ; nm, nuclear membrane ; p, a maSS OI CnonailOSOmeS, 



plasmosome. (( . ,, 



chromidia, etc., which it 

 appears to have attracted round itself (p. 191). 



(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 affinity for stains possessed by its constituents, 



