36 GENERAL SKETCH OF THE CELL 



stances. This is supported by the fact that in some forms of mitosis 

 the nucleolus is at the time of division actually cast out of the 

 nucleus into the cytoplasm, where it degenerates without further 

 apparent function. This seems to constitute decisive evidence in 

 support of Hacker's view as applied to certain cases; but without 

 further evidence it must remain doubtful whether it applies to 

 all. 1 



d. The ground-substance, nuclear sap, or karyolympJi, a clear sub- 

 stance occupying the interspaces of the network and left unstained 

 by most of the dyes that colour the chromatin, the linin, or the plas- 

 mosomes. By most observers the ground-substance is regarded as a 

 liquid filling a more or less completely continuous space traversed by 

 the nuclear network. By Biitschli, however, and some of his fol- 

 lowers the nucleus is regarded as an alveolar structure, the walls of 

 which represent the " network, "while the ground-substance corre- 

 sponds to the alveolar material. Nearly related with this is the view 

 of Reinke ( '94) that the ground-substance consists of large pale 

 granules of " lanthanin " or "oedematin." 



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 epithelial cells (Fig. i); sometimes extremely coarse and 

 irregular, as in leucocytes (Fig. 49); 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 network, but appears in the form of a thread closely similar to the 

 spireme-stage of dividing nuclei (cf. p. 65). The most striking case 

 of this kind occurs in the salivary glands of dipterous larvae (Chirono- 

 mus\ where, as described by Balbiani, the chromatin has the form of 

 a single convoluted thread, composed of transverse discs and termi- 

 nating at each end in a large nucleolus (Fig. 14, A). Somewhat simi- 

 lar nuclei (Fig. 14, B) occur in various epithelial cells of other insects 

 (Van Gehuchten, Gilson), and also in the young ovarian eggs of cer- 

 tain animals (cf. p. 273). In certain gland-cells of the marine isopod 

 Anilocra it is arranged in regular rosettes (Vom Rath). Rabl, fol- 

 lowed by Van Gehuchten, Heidenhain, and others, has endeavoured 

 to show that the nuclear network shows a distinct polarity, the 

 nucleus having a " pole " toward which the principal chromatin- 

 threads converge, and near which the centrosome lies. 2 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. Embryonic cells are in general 



1 Cf. pp. 126-130. 2 Cf. the polarity of the cell, p. 55. 



