26 



GENERAL SKETCH OF THE CELL 



the ground-substance has been regarded as a fluid or semi-fluid, but 

 recent researches by Reinke and others have thrown doubt on this 

 view, as described at p. 28. 



The configuration of the chromatic network varies greatly in dif- 

 ferent cases. It is sometimes of a very loose and open character, 

 as in many epithelia.1 cells (Fig. i) ; sometimes extremely coarse 



and irregular, as in leucocytes (Fig. 

 10) ; sometimes so compact as to 

 appear nearly or quite homogeneous, 

 as in the nuclei of spermatozoa and 

 in many Protozoa. In some cases 

 the chromatin does not form a net- 

 work, but appears in the form of a 

 thread closely similar to the spireme- 

 stage of dividing nuclei (cf. p. 47). 

 The most striking case of this kind 

 occurs in the salivary glands of dip- 

 terous larvae {CJiiroiiomus), where, as 

 described by Balbiani, the chromatin 

 has the form of a single convoluted 

 thread, composed of transverse discs 

 and terminating at each end in a 

 large nucleolus (Fig. w, A). Some- 

 what similar nuclei (Fig. 11, B) occur 

 in various glandular cells of other 

 insects (Van Gehuchten, Gilson), and 

 also in the young ovarian eggs of cer- 

 tain animals (cf. p. 193). In certain 

 gland-cells of the marine isopod Aui- 

 locra it is arranged in regular rosettes 

 (Vom Rath). Rabl, followed by Van 

 Gehuchten, Heidenhain, and others, 

 has endeavoured to show that the 

 nuclear network shows a distinct 

 polarity, the nucleus having a "pole" 

 towards which the principal chromatin-threads converge, and near 

 which the centrosome lies.^ In many nuclei, however, no trace of 

 such polarity can be discerned. 



The network may undergo great changes both in physical con- 

 figuration and in staining capacity at different periods in the life 

 of the same cell, and the actual amount of chromatin fluctuates, 

 sometimes to an enormous extent. Fmbryonic cells are in general 



Fig. 12. — An infusorian, Trachelo- 

 cerca, with diffused nucleus consi=ting of 

 scattered chromatin-granules. [Gruber.] 



^ ( 'f. the polarity of tlie cell, 



