66 



GENERAL PRINCIPLES OF ZOOLOGY. 



bounded externally by a nuclear membrane, easily isolated in large 

 nuclei. If little nuclear fluid be present, and the reticulum con- 

 sequently be coarse-meshed, the nucleus seems compact. If the 

 fluid be abundant, the nucleus appears vesicular. This is especially 



jchp 



FIG. 19. Vesicular nuclei with achromatic reticulum and different arrangements of 

 the chromatin and nucleolar substance, p, plastin (nucleolar substance); eft, 

 chromatin; chp, chromatin plus plastin land 2, nuclei of Actinosphcerium ; 3, 

 of Ceratium hirundella (after Lauterborn) ; 4, germinal vesicle of Unio (after 

 Flemming); 5, nucleus with many chromatin nucleoli. 



the case when the lines of the framework are separated by con- 

 siderable amounts of nuclear fluid (fig. 19, 4). 



The chromatin enters into close relations with a less stainable 

 substance, the plastin or paranuclein (also sharply distinct from 

 achromatin). In the nuclei of Protozoa plastin and chromatin 

 are usually intimately united, the first forming a substratum in 

 which the latter is embedded (chp). The united substances 

 are most frequently closely and regularly distributed as fine gran- 

 ules on the reticulum, so that the entire nucleus appears uni- 

 formly chromatic (fig. 18). More rarely the mixture collects into 

 one or more special bodies, the chromatic nucleoli (1, 2}. The 

 nucleolus is ordinarily a rounded body, more rarely branched 

 (fig. 19, 1). 



In the nuclei of the Metazoa there may occur the same intimate 

 mixture of plastin and chromatin (6). As a rule, however, the 

 plastin (apparently not the whole, but a surplus) is separate from 

 the chromatin. Thus there occur in the nuclei of many eggs 



