THE NUCLEUS 2 J 



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 



A description of the nucleus during division is deferred to the fol- 

 lowing chapter. 



2. Finer Structure of the Nucleus 



Many recent researches indicate that some at least of the nuclear 

 structures are 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 revolves itself into a definite number of rod-shaped 

 bodies known as chromosomes (Fig. 16), which split lengthwise as 

 the cell divides. These bodies arise as aggregations of minute 

 rounded bodies or microsomes to which various names have been 

 given (chromomeres, Fol ; ids, Weismann). They are as a rule 

 most clearly visible and most regularly arranged during cell-division, 

 when the chromatin is arranged in a thread (spireme}, or in separate 

 chromosomes (Figs. 7, D, 38, B] ; but in many cases they are dis- 

 tinctly visible in the reticulum of the "resting" nucleus (Fig. 39). 

 It is, however, an open question whether the chromatin-granules 

 of the reticulum are individually identical with those forming the 

 chromosomes or the spireme-thread. The larger masses of the 

 reticulum undoubtedly represent aggregations of such granules, but 

 whether the latter completely fuse or remain always distinct is 

 unknown. Even the chromosomes may appear perfectly homogene- 

 ous, and the same is sometimes true of the entire nucleus, as in the 

 spermatozoon. The opinion is nevertheless gaining ground that the 

 chromatin-granules have a persistent identity and are to be regarded 

 as morphological units of which the chromatin is built up. 2 



Heidenhain ('93, '94), whose views have been accepted by Reinke, 

 Waldeyer, and others, has shown that the "achromatic" nuclear net- 

 work is likewise composed of granules which he distinguishes as 

 lantJianin- or tf.i'j'r//;w;/rt//;/-granules from the basic/iromatin-gr&\-\\.\\es 

 of the chromatic network. Like the latter, the oxychromatin-granules 

 are suspended in a non-staining clear substance, for which he reserves 



1 See Chapter VII. - < T. Chapter VI. 



