THE NUCLEUS y^ 



(Schwarz), invisible until after treatment by reagents, which in sections 

 shows a finely granular structure and stains like the cytoplasmic sub- 

 stance, to which it is nearly related chemically (Figs. 7, 49). The 

 second constituent, a deeply staining substance known as chromatin 

 (Flemming), is the nuclear substance /^rr excellence, for in many cases 

 it appears to be the only element of the nucleus that is directly handed 

 on by division from cell to cell, and it seems to have the power to pro- 

 duce all the other elements. The chromatin often appears in the form 

 of scattered granules and masses of differing size and form, which are 

 embedded in and supported by the linin-substance (Figs. 7, 19). In 

 some cases the entire chromatin-content of the nucleus appears to be 

 condensed into a single mass which simulates a nucleolus ; for exam- 

 ple, in Spirogyra and in various flagellates and rhizopods (e.g. Acti- 

 nospJiceriiuii, Arcella) ; or there may be several such chromatin-masses, 

 as in some of the Foraminifera and in Noctihica. More commonly the 

 chromatin forms a more or less regular network intermingled with and 

 more or less embedded in the linin, from which it is often hardly dis- 

 tinguishable until the approach of mitosis, when a condensation of the 

 chromatin-substance occurs. 



In contradistinction to the other nuclear elements, chromatin is not 

 acted upon, or is but slowly affected, by peptic digestion. I : may thus 

 be easily isolated for chejiiical analysis, which shows it to consist 

 mainly of nucleiii, i.e. a compound in varying proportions of a complex 

 phosphorus-containing acid known as iincleinic acid, with albumi- 

 nous bodies such as histon, protamin, or in some cases albumin itself.^ 

 Upon this, as will be shown in Chapter VI., probably depends the pro- 

 nounced staining capacity when treated with the so-called " nuclear 

 stains " {e.g. hsematoxylin, methyl-green, and the basic tar-colours gen- 

 erally) from which chromatin takes its name. This capacity always 

 increases as the nucleus prepares for division, reaching a climax in the 

 spireme- and chromosome-stages, and it is also very marked in con- 

 densed nuclei such as those of spermatozoa. These variations are 

 almost certainly due to varying proportions in the constituents of the 

 nuclein, the staining capacity standing in direct ratio to the amount of 

 nucleinic acid. 



c. The nucleoli, one or more larger rounded or irregular bodies, 

 suspended in the network, and staining intensely with many dyes. 

 In some nuclei they are entirely absent. When present the nucleoli 

 vary in number from one to five or more ; and the number is often 

 inconstant in the same species of cell, and even varies in the same 

 cell with varying physiological conditions. In the eggs of some 

 animals, at certain periods of growth {e.g. lower vertebrates), the 

 nucleus may contain hundreds of nucleoli. An interesting case is 



1 See p. 334. 

 D 



