VISCIDITY OF PROTOPLASM 149 



be likened to a chan^'o Iroin a water-in-oil eniulsion to an oil-in- 

 water one (Chaj). IX.). 



The crystalloids, too, differ in their physical attributes from 

 similar salts in solution. A salt solution isotonic with a 0-9 per 

 cent, solution of sodium chloride exerts no osmotic effect either 

 positive or negative on the mammalian cells, but has an electrical 

 conductivity about five to thirty-five times as great as the cells. 

 That is. the cell offers a greater resistance to the passage of an 

 electrical current than its content of electrolytes would lead one to 

 expect. If now, the cell is injured so that its contents undergo 

 disintegration, its conductivity will approach that apparently 

 justified by its composition. The high electrical resistance of 

 living matter is due mainly to two factors — (1) the state in which 

 the water is held, and (2) the adsorption of a large proportion of 

 the electrolytes. (See also Chap. XXII., Blood.) 



The w^ater and a large proportion of the salts are dispersed 

 through an apparently homogeneous colloidal mass. From ultra- 

 microscopic examination one learns that the protoplasm varies in 

 viscosity from cell to cell and more markedly from animal to 

 animal. Some cells are almost liquid, as demonstrated by the 

 Vigour of the Brownian movement of the smaller granules in them, 

 while other cells appear to be decidedly viscous with sluggish 

 granular movements. The annelid egg exemplifies the foimer 

 and the sea urchin^s egg the latter state. 



The viscidity of protoplasm may be influenced by alterations in 

 the immediate environment. An excess of salts of monovalent 

 elements, e.g. Na, K, guanidine leads to an increase in liquidity, 

 while divalent cations produce the opposite effect. The presenta- 

 tion of a suitable mixture of mono- and divalent salts leads to the 

 optimum viscidity of any particular cell. Loeb showed that 

 Fundulus eggs, which were killed if placed in isotonic solutions of 

 the chlorides of either sodium or calcium, would thrive in a 

 definitely proportioned mixture of these two salts (cf. Salts and 

 surface action, Chap. IX.). 



The nucleus, as is mentioned above, is absolutely necessary for 

 the continued integrity of the cell. It is, in amoeba, a spherical 

 body, with a refractive index slightly higher than the cytoplasm 

 and contains a nucleolus which is still more highly retractile . It 

 also carries the chromosomes, the bearers or indicators of hereditary 

 characteristics. The nucleus is surrounded by a men^brane and is 

 generally considered to contain a reticulum. Experiment has 

 shown that the nucleoplasm reacts to salts much in the same way 

 as the cell itself. 



To sum up, the cell consists of three essential parts, (i) A mem- 



