[106] Warburg, O., _Sitzungsber. d. Heidelberger Akad. d. Wissnsch._, B. 1914. The question remains then: How can membrane formation or the alteration of the cortical layer underlying membrane formation cause an increase in the rate of oxidations? One possibility is that the iron (or whatever the nature of the catalyzer may be) exists in the cortex of the egg in a masked condition--or in a condition in which it is not able to act--while the alteration of the cortical layer makes the iron active. It might be that either the iron or the oxidizable substrate is contained in the lipoid layer in the unfertilized condition of the egg and that the destruction or cytolysis of the cortical layer brings both the iron and the oxidizable substrate into the watery phase in which they can interact. Another possibility is that the act of fertilization increases the permeability of the egg. This idea, which seems attractive, was first suggested and discussed by the writer in 1906.[107] He had found that when fertilized and unfertilized eggs were put into abnormal salt solutions, _e. g._, pure solutions of NaCl, the fertilized eggs died more rapidly than the unfertilized eggs and he pointed out that these experiments suggested the possibility that fertilization increases the permeability of the egg for salts. The reason for his hesitation to accept this interpretation was, that the fertilized egg is also more easily injured by lack of oxygen than the unfertilized egg and in this case the greater sensitiveness of the fertilized egg was obviously due to its greater rate of metabolism. Later experiments by the writer showed that the fertilized egg can be made more resistant to abnormal salt solutions if its development is suppressed by lack of oxygen or by KCN or by certain narcotics. With our present knowledge it does not seem very probable that lack of oxygen diminishes the permeability of the egg, but we know that it inhibits the developmental processes. Warburg has made it appear very probable that the fertilized egg is impermeable for NaOH and if this is the case it should also be impermeable for NaCl.[108] [107] Loeb, J., _Biochem. Ztschr._, 1906, ii., 87. [108] Unless the egg is left so long in the pure NaCl solution that its permeability is increased.