THE NUCLEUS 



37 



characterized by the large size of the nucleus ; and Zacharias has 

 shown in the case of plants that the nuclei of meristem and other 

 embryonic tissues are not only relatively large, but contain a larger 

 percentage of chromatin than in later stages. The relation of these 

 changes to the physiological activity of the nucleus is still imperfectly 

 understood. 1 



2. Finer Structure of tJie Nucleus 



A considerable number of observers 

 have raised the question whether the 

 nuclear structures may not be regarded 

 as aggregates of more elementary 

 morphological bodies, though there is 

 still no general agreement regarding 

 their nature and relationships. The 

 most definite evidence in this direction 

 relates to the chromatic network. In 

 the stages preparatory to division this 

 network resolves itself into a definite 

 number of rod-shaped bodies known 

 as chromosomes (Fig. 21), which split 

 lengthwise as the cell divides. These 

 bodies arise as aggregations of minute 

 rounded bodies or microsomes to which 

 various names h-ave been g\ve\\(c/iromo- 

 mcres, Fol ; ids, Weismann). They 

 are as a rule most clearly visible and 

 most regularly arranged during cell- 

 division, when the chromatin is ar- 

 ranged in a thread (spireme\ or in 

 separate chromosomes (Figs. 8, D, 53, 

 B): but in many cases they are dis- Fi g- I 5- A" infusorian, Tracheio- 



. . ....,., , f . cerca, with diffused nucleus consisting of 



tmctly Visible 111 the retlCUlum Of the sca ttered chrornatin-granules. [GRUBER.] 



"resting" nucleus (Fig. 54). It is, 



however, an open question whether the chromatin-granules of the 

 reticulum are individually identical with those forming the chromo- 

 somes or the spireme-thread. The larger masses of the reticu- 



1 Both chromatin-granules and nucleoli have been seen in a considerable number of living 

 cells (Fig. 9). Favourable objects for this purpose are according to Korschelt ('96) the silk- 

 glands of caterpillars, where the whole nucleus may be seen to be filled with fine granules 

 ("microsomes"), among which are scattered many larger granules (" macrosomes"). The 

 later studies of Meves ('97, i) make it probable that the latter are true nucleoli and the for- 

 mer chromatin-granules. Korschelt, however, regards the " macrosomes " as composed of 

 chromatin and the "microsomes" as representing the so-called "achromatic substance." 



