22 NUCLEAR DIVISION 



process is designated mitosis or indirect nuclear division. 1 All 

 stages can be seen in a thin longitudinal section through the 

 growing point of a higher plant (cf. Fig. 9); but in order to 

 observe them readily, it is necessary to employ material which 

 has been carefully preserved and suitably stained (see Appen- 

 dix VI). 



The nucleus is usually oval in form (Figs. I, N ; 10, A), and, 

 when not actually dividing, is spoken of as a resting nucleus, a 

 term, however, which is apt to be misleading, since in this state 

 it is probably just as active. Immediately within the nuclear 

 membrane (p. 3) there is a more or less irregular network of 

 deeply-stained substance, the chromatin (Fig. 10, A). The 

 nucleoli (n), which are likewise deeply stained and consist of 

 similar material, are situated just within this reticulum. The 

 whole of the central region is occupied by the unstained nuclear 

 sap, a denser peripheral portion of which contains the chromatin 

 network. 



The first change, indicating that division is about to occur, 

 is a gradual simplification of the chromatin network, owing to 

 the disappearance of some of the cross-connections and the 

 closing up of some of the meshes. Later the chromatin network 

 appears to consist of a number of irregular, ill-defined segments 

 still exhibiting attachment to one another, especially near the 

 ends. In the next stage, as a result of further concentration 

 and disappearance of all except the terminal chromatin-connec- 

 tions, there remains a single thread exhibiting a split along its 

 whole length, and irregularly coiled around the nucleolus or 

 nucleoli (Fig. 10, B). This is known as the spireme stage. 



Soon after this the nucleoli disappear, their chromatin prob- 

 ably having been absorbed into the thread. The latter now 

 contracts somewhat, thus becoming thicker, and breaks up into 

 segments or chromosomes (Fig. io,CandD, ch.). The number 

 of these has been found to be practically constant for the vege- 

 tative cells of any particular species. 



Meanwhile the cytoplasm surrounding the nucleus becomes 



denser, and numerous streaks become apparent, radiating from 



each end of the cell towards the nucleus (Fig. 10, C, D), whose 



membrane has by this time practically disappeared. The radia- 



1 The antiquated term karyokinesis is now rarely employed. 



