

23.] 



CHAPTER I. THE CELL. 



97 





simply in the constriction of the nucleus into two halves (Fig. 

 48). " 



Indirect or mitotic division of the nucleus is generally associated 

 with cell-division ; in ccenocytes, however, the indirect nuclear 

 division is not followed by cell-division, but each division results 

 simply in a corresponding increase in the number of protoplasmic 

 units present (energids, see p. 90). The description of this 

 mode of nuclear division is included in the account of cell- 

 division (p. 118). 



c. The Centrospliere, though a perfectly independent body, is 

 always closely associated with the nucleus. It is spherical in 

 form, and consists of a central particle (the centrosome), surrounded 

 by a considerable hyaline area, limited by a granular outline. It 

 undergoes division in connection 



with the division of the nucleus, as 

 described in the section dealing with 

 that subject. It appears that there 

 are normally two of these bodies 

 associated with the nucleus, lying 

 close together (Fig. 49) in the kino- 

 plasm. 



d. The Plastids are differentiated 

 rtions of the protoplasm which, 

 e the nucleus, are not formed de 



novo, but multiply by division. Their 

 form varies widely. Structurally, 

 they seem to consist of a ground- 

 substance with imbedded fibrils, 

 denser at the surface. Chemically, they have been described as 

 consisting of two proteid substances ; the one, termed chloroplastin, 

 constitutes the ground-substance, and seems to be similar to the 

 cytoplastin of the cytoplasm; the other, termed metaxin, is the 

 material of the fibrils, and appears to be altogether peculiar to the 

 plastids. 



The plastids may either be colourless, when they are termed 

 leucoplas tids ; or coloured, when they are termed cJiromatophores. 

 The chromatophores are distinguishable as chloroplastids, when they 

 contain the green colouring-matter chlorophyll; or as chromoplastids 

 when they contain no chlorophyll, but some other colouring- 

 matter. Plastids are not found in the Fungi, nor, apparently, in 

 the Cyanophycese among the Algae. 



FIG. 49. Young pollen-grain of 

 Lilium Martagon, showing, c two cen- 

 trospheres, and n the resting nucleus. 

 (After Guignard : x 750.) 



