^^ GENERAL SKETCH OF THE CELL 



that of the subcutaneous gland-cells of Pisciola, the nuclei of which 

 contain in early phases of secretion but a single nucleolus. During 

 growth of the cell the nucleolus fragments, finally giving rise to 

 several hundred nucleoli which then appear to migrate out into the 

 cytoplasm, leaving but a single nucleolus to repeat the cycle. ^ 



The bodies known as nucleoli are of at least two different kinds. 

 The first of these, the so-called true nucleoli or phuinosomcs (Figs. 6, 

 8, B, 13), are of spherical form, and are shown by the staining 

 reactions to differ widely from chromatin, being in general sharply 

 stained by dyes which, like eosin, orange or acid fuchsin, stain the 

 linin and the general cytoplasm. The plasmosomes sometimes seem 

 to have no envelope, but in many cases {e.g. in leucocytes) are 

 surrounded by a thin layer that stains Hke chromatin. Nucleoli of a 

 quite different type are the *' net-knots " (Netzknoten), chromatin- 

 nucleoli, or karyosojucs, which agree in staining reaction with chro- 

 matin and are doubtless to be regarded as merely a portion of the 

 chromatin-network (Figs. 8, 49). These are sometimes spherical, 

 more often irregular (Fig. 8), and often are hardly to be distinguished, 

 except in size, from nodes of the chromatin-reticulum.''^ The relations 

 between these two forms of nucleoli are far from certain, and the 

 variations in staining reaction shown by true nucleoli render it not 

 improbable that intermediate forms exist which may represent an 

 actual transition from one to the other.-*^ In many of the Protozoa, 

 as described beyond, the ''nucleolus" is shown by its behaviour 

 during mitosis to be comparable with an attraction-sphere or centro- 

 some ('Muicleolo-centrosome," Keuten); and even in higher forms 

 there are some cells in which the centrosome is intranuclear 



(Fig. 148). 



There is good reason to believe that the chromatin-nucleoli are 

 merely more condensed portions of the chromatin-network. since 

 during cell-division they have the same history as the remaining 

 portion of the chromatin-substance.* The nature of the true nucleoli 

 is still imperfectly known. By some observers, including Flemming, 

 O. and R. Hertwig, and Carnoy, they have been regarded as store- 

 houses of material (para-nuclein, plastin) which contributes to the 



^ Montgomery, '98, 2. 



2 Flemining first called attention to the chemical difference between the true nucleoli and 

 the chromatic reticulum ('82, pp. 138, 163) in animal-cells, and Zacharias soon afterward 

 studied more closely the difference of staining reaction in plant-cells, showing that the 

 former are especially coloured by alkaline carmine solutions, the latter by acid solutions. 

 Other studies by Carnoy, Zacharias, Ogata, Rosen, Schwarz, Heidenhain, and many others 

 show that the medullary substance (pyrenin) of true nuclei is coloured by acid tar-colours and 

 other plasma stains, while the chromatin has a special affinity for basic dyes. Cf. p. 337. 



3 For very full review of the literature of the nucleoli see Montgomery ( '98, 2). 



4 Cf. p. 67. 



