38 DYNAMICS OF LIVING MATTER 



division and protoplasmic motion, rhythmic contractions, etc., depend 

 upon alternating gelations and liquefactions. It is, however, useless 

 to discuss such possibilities until more definite proofs of their real ex- 

 istence have been furnished. Such proofs thus far exist only in regard 

 to membranes of precipitation. 



4. THE FORMATION OF SURFACE FILMS AND TRAUBE'S MEMBRANES 



OF PRECIPITATION 



It is a general rule that every free cell is surrounded by a solid film. 

 The pseudopodia of many Infusorians could not exist were they entirely 

 liquid. Liquid circular cylinders begin to fall apart into droplets as 

 soon as their height becomes greater than the periphery of their base. 

 The length of the pseudopodia of rhizopods is, however, very often a 

 multiple of their circumference. As the interior of the pseudopodia 

 shows phenomena of streaming, the solid part of the pseudopodia can 

 only be at their surface. Such solid surface films may be exceedingly 

 thin, according to Quincke's observations. 



Ramsden has recently shown why masses of protoplasm must form 

 solid films at their surface. He had formerly observed that the white 

 of a hen's egg can be caused to coagulate by mere mechanical agita- 

 tion.* The explanation of this fact was subsequently found in the fur- 

 ther observation, that without any evaporation at the free surface of 

 the protein solutions, solid or extremely viscous films are formed very 

 rapidly. f If such solid particles be removed from the surface, i.e. by 

 mechanical agitation, new particles will come to the surface and form 

 membranes. This is in harmony with what was stated earlier in regard 

 to the gathering of contaminating particles at the surface between two 

 media. 



What has been said here with reference to the formation of solid 

 films at the surface of free cells, may also hold with regard to the for- 

 mation of solid films at the surface of nuclei. 



Traube has shown that where two liquid colloids come in con- 

 tact, solid membranes may be formed. He investigated the mechanism 

 of the formation of the membranes of plant cells and was led to the 

 conclusion that the formation of these membranes, and the peculiarity 

 of the cell to grow, depend upon a simple physical process. Certain 

 colloids form a precipitate where they come in contact with each other, 

 and this precipitate is impermeable for either colloid. The precipitate 

 must therefore assume the shape of a thin film, which prevents the 



* Ramsden, Archiv ftir Anatomic und Physiologic, Physiologische Abteilung, p. 517, 1894. 

 t Ramsden, Zeitsch. fur physik. Chemie, Vol. 47, p. 336, 1904. 



