l^ROTOPI.ASM AND COLLOIDS 



189 



eosity. Tlie low \;ilues for protoplasmic 

 viscosity wliidi jirc ()l)taine(l Avith the cen- 

 trifuge method liave been checked by deter- 

 minations made from a study of Brownian 

 movement. These values pertain to the 

 Jiyaline material in which granules are 

 suspended. Since oi-dinarily there is a 

 heavy concentration of granular material, 

 the viscosity of tlie whole mass is several 

 times greater than that of the hyaline fluid 

 alone. And indeed in some cases in which 

 the protoplasm is very densely packed with 

 granules (for example, in Paramecium), 

 the viscosity of the protoplasm as a whole 

 may be many times greater than that of 

 the hyaline fluid. For I'eferences to litera- 

 ture on protoplasmic viscosity, see Heil- 

 brunn (1928; 1937). 



Earlier studies of protoplasmic viscosity 

 were concerned only with the main mass of 

 the protoplasm in the interior of the cell. 

 Later we became interested in the cortex 

 of the cell. In the unfertilized sea-urchin 

 egg the cortex is so thin and delicate that 

 it remained unnoticed until described by 

 Moser (1939). Some minutes after fertili- 

 zation the cortex becomes much thicker so 

 that it is readily visible. In both unfertil- 

 ized and fertilized Arhacki eggs the cortex 

 is so stiff that even high centrifugal forces 

 are incapable of moving granules through 

 it, unless it first be made fluid. In Anioeha 

 proieus the cortex is much thicker than in 

 the sea-urchin egg. Moreover, its viscosity 

 can be tested by strong centrifugal forces, 

 for such forces are capable of pushing 

 granules through it. In my laboratory we 

 have made a number of studies of the vis- 

 cosity of the cortex or plasraagel of ameba. 

 One of the significant facts that we liave 

 observed is that the high viscosity of the 

 cortex depends on the ])resence of calcium 

 ion. If this ion is replaced by sodium or 

 potassium, or even by magnesium, the vis- 

 cosity of the cortex decreases ; a similar de- 

 crease occurs if the amebae are immersed 

 in oxalate solutions, which can be assumed 

 to remove calcium from the cortex. 



Typicall}'', as has already been noted, the 

 protoplasm in the interior of a resting cell 

 is fluid. Suspended in the protoplasmic 



fluid are innumerable granular particles. 

 The fact that these tend to remain discrete 

 and se[)arate from each other is an indica- 

 tion that they bear an electric charge. It 

 is of some importance for us to know the 

 sign of this charge on the protoplasmic 

 granules. And yet very few attempts 

 liave been made to determine whether the 

 charge is positive or negative. Nor is such 

 a study as simple as it might .seem. So 

 sensitive is protoplasm to an electric cur- 

 rent that almost any passage of electricity 

 through it is apt to cause destruction of 

 the cell. Our recent experiments (Heil- 

 brunn and Daugherty 1939) indicate that 

 tlie charge on the protoplasmic inclusions 

 witliin ameba protoj^lasm is positive. This 

 may seem strange to students of inanimate 

 colloids and proteins, especially if the pH 

 of the cell is as high as some authorities 

 claim. However, it is in accord with an 

 early observation of Hardy on the cyto- 

 plasm of the cells of the onion root tip, as 

 well as with deductions made from centri- 

 fuge viscosity measurements. Apparently 

 the positive charge on protoplasmic inclu- 

 sions is due to the presence of carbonic acid 

 within the cell. If the protoplasm in ameba 

 is made alkaline, the positive charge is neu- 

 tralized or reversed. Moreover, in the leaf 

 cells of the water plant Elodea, it is only 

 when carbon dioxide is present that the cell 

 chloroplasts are positively charged. Dur- 

 ing active photosynthesis, when carbon 

 dioxide tends to be used up, the chloro- 

 plasts are often charged negatively. 



AVhereas the inclusions within the cyto- 

 plasm, and presumably also the nncellae of 

 the protoplasmic colloid, are charged posi- 

 tively, the chromatin of the nucleus carries 

 a negative charge. This has long been 

 known for plant cells, but it has only re- 

 cently been shown for any animal cell. In 

 a study of the salivary glands of fly larvae, 

 Churney and Klein (1937) found that the 

 chromatin migrated to the anode. How- 

 ever, the nucleus as a whole moves to the 

 cathode, a fact which indicates there is a 

 positive charge on the cytoplasmic colloids 

 surrounding the nuclear membrane. I 

 have said that Churnev and Klein were 



