192 
F. F. Blackman. 
tates. 1 Senility brings on spontaneous coagulation and so do 
certain physical and chemical treatments. Heat, pressure and 
deformation by plasmolysis are agents of the first class; and salts 
of heavy metals, acids, alcohol, etc., represent the second class. 
The way in which these agents act is such as to indicate that 
proteids are the dominant constituent of the plasma membrane. As 
its outer surface does not mix with water, the membrane cannot 
be colloid particles distributed in a water medium, but the con¬ 
tinuous medium must be something else which can imbibe water 
without dissolving in it—probably a proteid with water imbibed in 
it, and also with particles of water distributed through it as an 
emulsion-colloid. 
On coagulation the proteids aggregate to form large particles 
and the water runs together to form fine canals through which 
non-selective permeation readily takes place. 
On general principles, penetration of such a living membrane 
by any given substance must depend on the solubility of the 
substance in the continuous medium and it would seem that it 
should not be entirely impenetrable to any water-soluble crystal¬ 
loid. 3 
Lepeschkin draws attention to an important flaw in the 
evidence held to prove the impenetrability of many dyes soluble in 
water. Unless such a dye unites with something inside the cell 
and is therefore cumulatively stored there can be no visual evidence 
as to whether it penetrates or not, for the colour of the unconcen¬ 
trated solution is too pale to be visible in the cell. 
1 A number of precipitates of colloid substances are, when first 
formed, spherical drops of fluid containing much water, and 
they only become solid granular aggregates after some time. 
Quincke showed that copper ferrocyanide when first pre¬ 
cipitated is liquid, but it sets solid in a few seconds. 
Lepeschkin added 90% alcohol to 20% ammonium sulphate 
and found that a cloud of drops separates, crystallising 
only after some hours. Like the setting of plasma this 
solidification is hastened by mechanical stress. Similar 
drops of liquid precipitate are formed when saturated am¬ 
monium sulphate is added to 20% albumose, and these drops 
coagulate spontaneously in time. These drops of liquid pro¬ 
teid behave not unlike protoplasts in that they become 
vacuolated in salt-free albumose solution by taking up water. 
They then show selective permeability in that they can be 
shrunk to almost nothing (plasmolysed) by immersion in 
strong sugar solution, while saturated potassium nitrate 
penetrates too fast to plasmolyse them. 
2 New important relations between the size of their colloid par¬ 
ticles and the uptake of aniline dyes by the protoplast are 
indicated in a recent paper by Ruhland “ Die Plasmahaut 
als Ultra-filter bei der Kolloidaufnahmc.” Ber. deut. bot. 
Ges., Bd., XXX, April, 1912, 
