GENERAL ANATOMY 



57 



If the fluid be abundant, the nucleus appears vesicular. This is espe- 

 cially the case when the lines of the framework are separated by consider- 

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



The chromatin enters into close relations with a less stainable element, 

 also distinct from achromatin, the plaslin, {paranuclein, p). In the 

 protozoan nuclei plastin and chromatin are usually intimately united, 

 the first forming a substratum in which the latter is imbedded (clip). 

 The united substances are most frecjuently closely and regularly dis- 

 tributed as tine granules on the reticulum, so that the entire nucleus 



ch p 



4 56 



Fig. 19. — ^Vesicular nuclei with achromatic reticulum and different arrangements 

 of the chromatin and nucleolar substance, p, plastin (nucleolar substance) ; ch, 

 chromatin; clip, chromatin plus plastin. i and 2, nuclei of Actinosphccrium; 3, of 

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

 5, nucleus with many chromatin nucleoU. 



appears uniformly chromatic (fig. 18). More rarely the mixture collects 

 into one or more special bodies, the chromatic nucleoli {amphinucleoli, 

 caryosomes, fig. 19, i, 2). The nucleolus is ordinarily a rounded body, 

 more rarely branched (fig. 19, i). 



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

 ture 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 nucleoli which consist of two 

 distinguishable parts, the one containing chromatin, the other chromatin 

 free (4). Usually in tissue cells only the plastin has the form of 



