60 



GENEliAL PRINCIPLES OF ZOOLOGY. 



bounded externally by a nuclear membrane, easily isolated in large 

 nuclei. 11 little nuclear fluid be i)resent, and the reticulum con- 

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

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



clip 



j chp 



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

 tlie cliromatin and nucleolar substance, /i, plastin (nucleolar substance); ch, 

 clironiatin; r/ip, chromatin plus plastin. land 2, nuclei of Acthin.-;ji]iiTriHm : 3, 

 of Crrnliuin ItiriuKleUa (after Lauterborn) ; 4, germinal vesicle of Uni<i (after 

 Flemming); .5, nucleus witli many clironiatin nucleoli. 



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

 siderable amounts of nuclear fluid (fig. 10, ^). 



The cliromatin enters into close relations with a less stainable 

 substance, the plastin or parannclein (also sharply distinct from 

 achromatin). In the nuclei of Protozoa ])lastin and chromatin 

 are nsually intimately united, the first forming a sulistratum in 

 which the latter is embedded (rlip). TIiq nuited substances 

 are most frequently closely and regularly distributed as fine gran- 

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

 formly chromatic (fig. IS). More rarely tlte mixture collects into 

 one or more special Ixjdies, tlie cliromatic nucleoli (1, '2). The 

 nucleolus is ordinarily a ri)U]ided liody, more rarely branched 

 (fig. 10, I). ' 



In tlie nuclei of tlu^ Metazoa tliere may occur the same intimate 

 mixture of plastin arid chromatin (6'). As a rule, howeyer, the 

 ])lastin (a]ii)arently not tlie whole, but a surplus) is separate from 

 the clironuitin. "^riius tliere occur in the nuclei of many eggs 



