Il6 J. F. MCCLEXDOX. 



part of the solvent away from the second and increases the con- 

 centration of the second substance. This may explain the effect 

 of a harmless substance in increasing the toxicity of a poison. 

 Schnerlcn 1 observed that a solution of phenol below the threshold 

 of toxicity for certain bacteria is rendered toxic by adding NaCl. 

 Stockard showed that the toxicity of pure solutions of salts on fish 

 eggs is increased by the addition of sugar, although the total 

 osmotic pressure of the mixture is less than that of the normal 

 medium. 2 



Just as Traube's precipitation membranes are absolutely 

 impermeable to certain substances, so do living cells show this 

 selective permeability. For instance, the vacuole fluid or cell 

 sap of certain plant cells contains colored substances which do 

 not diffuse into the protoplasm surrounding the vacuoles. If a 

 cell be placed in a solution of the pigment, the protoplasm 

 remains colorless. If the protoplasm be squeezed out of the 

 cell into a solution of the pigment, it does not invariably become 

 stained. However, if the cell is injured in certain ways, or 

 dies from any cause, the pigment diffuses out of the vacuoles 

 into the protoplasm and thence into the surrounding medium. 

 \Ye might conclude that the protoplasm in general is imperme- 

 able to the color, but at death it becomes permeable. On the 

 other hand, Pfeffer 3 gives evidence for the existence of a mechani- 

 cal membrane on the surface of the cell and lining the vacuoles. 

 De Vries 4 placed cells into 10 per cent. KNOa solution colored 

 with eosin. The plasma membrane and granular plasm died 

 and stained long before any dye entered the vacuoles. How- 

 ever, the granular plasm may have absorbed all the dye, thus 

 preventing its entrance for some time, without the necessity of 

 any resistance of the vacuole membrane. Since protoplasm may 

 be squeezed out in the form of droplets and still appears to be 

 surrounded by membranes, Pfeffer concluded that the membrane 

 was formed by the contact of the protoplasm with the medium 



i Arch. exp. Path.. 1X96, XXXVII.. 84. 



- However the. NaCl in Schnerlen's and sugar in Stockard's experiment may 

 have increased the permeability to the toxic substances, as discussed in later 

 chapters. 



" I'Han/cnphysiologie." 



4 Jahrh. wiss. Bol.. 1885, XVI., 465. 



