296 ON THE INTERNAL FORM [ch. 



uniform surface-tension, and through whose boundary-film diffusion 

 is taking place; but here, owing to the small difference between the 

 fluid constituting and that surrounding the drop, the surface-tension 

 equihbrium is somewhat unstable; it is apt to vanish, and the 

 rounded outhne of the drop disappears, Uke a burst bubble, in a 

 moment. 



If, on the other hand, the substance of the cell acquire a greater 

 soUdity, as for instance in a muscle-cell, or by reason of mucous 

 accumulations in an epithehum cell, then the laws of fluid pressure 

 no longer apply, the pressure on the nucleus tends to become 

 unsymmetrical, and its shape is modified accordingly. Amoeboid 

 movements may be set up in the nucleus by anything which disturbs 

 the symmetry of its own surface-tension; and where "nuclear 

 material" is scattered in small portions throughout the cell as in 

 many Rhizopods, instead of being aggregated in a single nucleus, 

 the simple explanation probably is that the "phase difference" (as 

 the chemists say) between the nuclear and the protoplasmic substance 

 is comparatively shght, and the surface-tension which tends to keep 

 them separate is correspondingly small*. 



Apart from that invisible or ultra-microscopic heterogeneity 

 which is inseparable from our notion of a "colloid," there is a 

 visible heterogeneity of structure within both the nucleus and the 

 outer protoplasm. The former contains, for instance, a rounded 

 nucleolus or "germinal spot," certain conspicuous granules or 

 strands of the peculiar substance called chromatin*)*, and a coarse 

 mesh work of a protoplasmic material known as "linin" or achro- 

 matin; the outer protoplasm, or cytoplasm, is generally believed 

 to consist throughout of a sponge-work, or rather alveolar mesh- 

 work, of more and less fluid substances; it may contain "mito- 

 chondria," appearing in tissue-cultures as small amoeboid bodies; 

 and lastly, there are generally to be detected (in the animal, rarely 

 in the vegetable kingdom) one or more very minute bodies, usually 

 in the cytoplasm sometimes within the nucleus, known as the 

 centrosome or centrosomes. 



* The elongated or curved "macronucleus" of an Infusorian is to be looked 

 upon as a single mass of chromatin, rather than as an aggregation of particles in 

 a fluid drop, as in the case described. It has a shape of its own, in which ordinary 

 surface-tension plays a very subordinate part. 



•j- First so-called by W. Flemming, in his Zellsubstanz, Kern und Zelltheilung, 1882. 



