STRUCTURE.] NAGELl's THEORY, 37 



cantia.) The contents of the nucleal vesicle run through a 

 series of alterations. These consist, morphologically, in this : 

 from the amorphous mucilage, granules are formed, which 

 lie in a watery fluid ; chemically, in this : from a mixture of 

 mucilage and gum are produced mucilage, starch, and chloro- 

 phyll-granules, oil-globules, and colouring matter. The 

 vital processes, the fashioning, (Gestaltung), and the propa- 

 gation of the nucleal vesicle, agree in general with those of 

 the cell, and it must in like manner be regarded as an 

 individual organism. 



" The nucleus, when fully developed, contains one or more 

 nucleoli. This structure I have pointed out in free and 

 parietal nuclei in the Algse, Fungi, Floridese, Hepaticse, 

 Musci, Characese, Equisetacese, Filices, Lycopodiacese, and 

 Phanerogamia. In many nuclei, however, the nucleolus has 

 not yet been detected. This is most striking in the Fu- 

 coideae, where the nuclei are quite large, and can readily be 

 set free. The investigation is rendered very difficult in 

 these by the peculiarity of the cell-contents. Moreover, the 

 nucleal vesicles are for a long time densely filled with mu- 

 cilage, and in a condition that, even in the Phanerogamia, 

 usually renders the observation of the nucleoli impossible. 

 All other nuclei without nucleoli (and to these belong only 

 the nuclei of several Fungi and most Algse) are small, or the 

 opacity of the cell-contents renders an accurate examination 

 impracticable. At this time I know of no nucleus in which 

 the absence of the nucleolus has been distinctly made out ; 

 I am inclined, therefore, to set forth generally, and to claim 

 as an essential character of the nucleal vesicle, that it con- 

 tains one or more nucleoli. Although the old and meta- 

 morphosed nuclei in Cystoseira, which sometimes possess 

 merely a membrane with transparent contents, certainly no 

 longer contain nucleoli, this does not argue against the 

 assumption ; since in old cells, in which the chemical changes 

 of the contents are completed, the nucleus also is mostly 

 absorbed. 



" The assumption that the nucleus has the complex struc- 

 ture above indicated, might indeed be an improbable one as 

 to the small point-like nuclei of those cellules, in which the 



