Protoplasm by Aid of Microdissection . 285 
I have spoken only of surface forces. That these are sufficient to 
satisfy all membrane activity is unlikely. The state of equilibrium which 
is established in the membrane, and to perhaps a less degree in the hyalo- 
plasmic border, is upset by extension and contraction. What forces are 
concerned in re-establishing this equilibrium we can only conjecture. But 
in considering them one must not lose sight of the fact that the organism is 
full of regulatory reactions, and an explanation of membrane formation 
based simply on surface phenomena is undoubtedly inadequate. 
Comparison of the Plasma-membrane with Precipitation Membranes , 
Just how closely the plasma-membrane is to be compared with those 
precipitation membranes which are formed when two substances such 
as CaCl 2 and K 2 C 0 3 , or CuS 0 4 and K 4 Fe(CN) 6 are brought together 
is a matter open to discussion. These precipitation membranes and the 
plasma-membrane have much in common, but also differ in some very 
fundamental properties. (I am adverse to making any distinction between 
the living and the non-living which tends to ascribe to the living any 
properties not possessed by the non-living, other than such as result from 
the great complexity of the former. This complexity, or organization 
as it is sometimes called, is, however, a distinguishing characteristic of the 
living, and we have as yet no satisfactory explanation of it. The living 
membrane manifests this organization— that is, it exhibits certain self- 
regulatory reactions not possessed by the non-living membrane.) 
Comparison of the plasma-membrane and the classical Traube precipi- 
tation membrane of Cu 2 Fe(CN) 6 is misleading in at least one respect. The 
former is a reversible gel, 1 the latter an irreversible one ; the former grows 
by reversion to a sol and the addition to the surface of colloidal particles 
from the interior; the latter (the Traube membrane) grows by rupture 
of the surface and the formation of a new membrane. Hober (20, p. 64) 
says, ‘ The plasma-membrane must probably be a thin surface layer com- 
parable to a Traube precipitation membrane’. This may be true in a very 
broad and general way, but Hober’s further statement, that as the Traube 
membrane grows so does the plasma-membrane, is not true if the comparison 
is to the method of growth by rupture and subsequent healing which is 
characteristic of the typical Traube membrane. Growth of the plasma- 
membrane is accomplished by a deposition of colloidal particles (and 
possibly substances in molecular dispersion) at the surface between the 
particles already at the surface which become somewhat separated as 
the area of the fluid membrane increases. (The deficit of colloidal particles 
just below the surface, which results from this surface deposition, is 
replenished from within by diffusion.) A better analogy is to be had in the 
remarkable precipitation membrane produced by bringing HC 1 and 
1 Bechhold’s statement (4, p. 56), ‘ We may describe membranes as irreversible gels \ if in 
reference to living plasma-membranes, is quite untenable. 
