720 RALPH S. LILLIE 
In a resting cell like the unfertilized egg there are two semi- 
permeable membranes, the plasma-membrane bounding the entire 
cell and the nuclear membrane bounding the inner surface of 
the cytoplasm. The cytoplasm between these surfaces is typi- 
cally homogeneous‘! and may hence be regarded as freely perme- 
able to diffusible crystalloid substances, including ions. This is 
indicated by the fact that diffusible coloring substances, like the 
pigment of Arbacia eggs, tend to become uniformly distributed 
throughout the cytoplasm, but meet with barriers at nuclear 
and plasma membranes; Héber has recently adduced physico- 
chemical evidence indicating that ions are free to diffuse within 
the cytoplasm, though they encounter marked resistance at the 
plasma-membrane.*2 The semi-permeability of both nuclear and 
plasma membranes is evidenced by the difference between the 
inorganic salt-content of the cytoplasm on the one side and of 
both external medium and nucleus on the other. Numerous other 
proofs for the essential semi-permeability of the plasma-mem- 
brane are well known; Macallum’s researches constitute the best 
proof for a similar condition in the nuclear membrane.“ The 
inappreciable; yet the P. D. is 0.51 volt. Particles of suspended quartz show a 
.P.D. against the water of 0.044 volt. Cf. Freundlich: Kapillarchemie, p. 234. 
It should also be noted that if the protoplasm serves in any way as a source of 
hydrogen-ions there will be a potential-difference at the surface even though the 
actual H-ion concentration within the protoplasm is extremely low. To account 
for the P.D. at the surface of the zine plate it is not necessary to assume that 
Zn-ions exist as such in the metal; all that is required is that the zinc which passes 
beyond the surface into the solution should be ionized. There is thus no necessary 
discrepancy between the present hypothesis and the view which regards proto- 
plasm as practically neutral in reaction. This, however, is demonstrably not 
always the case; contracting muscle (e.g.) may be distinctly acid to litmus, indi- 
cating a H-ion concentration exceeding 10-'n . 
“1 This statement relates particularly to cells about to Aaeaae by mitosis. Such 
cells typically lack ‘differentiation;’ i.e., the colloidal as well as the crystalloidal 
material usually shows a uniform distribution. Egg-cells with their stored masses 
of inert food material or yolk are frequent exceptions. 
* Hoéber: International Physiological Congress, September 1910; Archiv fiir 
die gesammte Physiologie, 1910, vol. 133, p. 237. 
48 Cf. Macallum: Ergebnisse der Physiologie, 1908, vol. 7, p. 552. Hamburger 
finds the nuclei of intestinal and tracheal epithelial cells decidedly less permeable 
to NaCl than the cell-bodies of the same cells; nuclei of other epithelia (bladder 
and oesophagus) are also impermeable to NaCl. Cf. Osmotischer Druck und 
Ionenlehre, vol. 3, pp. 8 seq. Roémkes finds the nuclei of liver cells similarly im- 
permeable (cf. Biochemische Zeitschrift, 1908, vol. 14, p. 254). 
