166 ON THE INTERNAL FORM AND [ch. 



cell, or by reason of mucous accumulations in an epithelium cell, 

 then the laws of fluid pressure no longer apply, the external 

 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 the cases, as in many Rhizopods, 

 where "nuclear material" is scattered in small portions throughout 

 the cell instead of being aggregated in a single nucleus, are probably 

 capable of very simple explanation by supposing that the "phase 

 difEerence" (as the chemists say) between the nuclear and the 

 protoplasmic substance is comparatively slight, and the surface- 

 tension which tends to keep them separate is correspondingly 

 small*. 



It has been shewn that ordinary nuclei, isolated in a living 

 or fresh state, easily flow together; and this fact is enough to 

 suggest that they are aggregations of a particular substance rather 

 than bodies deserving the name of particular organs. It is by 

 reason of the same tendency to confluence or aggregation of 

 particles that the ordinary nucleus is itself formed, until the 

 imposition of a new force leads to its disruption. 



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, for instance, contains a rounded 

 nucleolus or "germinal spot," certain conspicuous granules or 

 strands of the pecuHar substance called chromatin, and a coarse 

 meshwork of a protoplasmic material known as "Hnin" or achro- 

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

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

 work, of more and less fluid substances ; and lastly, there are 

 generally to be detected one or more very minute bodies, usually 

 in the cytoplasm, sometimes within the nucleus, known as the 

 centrosome or centrosomes. 



The morphologist is accustomed to speak of a "polarity" of 



* 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. 



