PHYSIOLOGICAL EFFICIENCY OF ACIDS 143 
a rule no membrane formation. This means that the latter 
must depend upon the diffusion of the undissociated acid mole- 
cule. These facts support the idea that only undissociated 
molecules and not ions diffuse into the cell. This should 
be taken into consideration by those who maintain that the 
effect of fertilization consists in an increased permeability of 
the egg for ions. 
A fact which agrees well with the above statement is that 
carbonic acid is especially effective for membrane forma- 
tion; I discovered this in my earlier experiments,’ and it has 
been confirmed by Godlewski.? Carbonic acid is a very weak 
acid. 
Now it is found that the eggs form no membranes upon 
treatment with the lower fatty acids while they are in the acid, 
but only after they are transferred to normal sea-water. ‘This 
holds for formic, acetic, propionic, butyric, valerianic, and 
capronic acids. Heptylic, caprylic, nonylic, and caprinic acids, 
however, behave otherwise, for the eggs form membranes while 
they are in the solution of these higher acids. The explanation 
of this, I believe, is to be found in the fact that the higher mono- 
basic fatty acids are but little soluble in water and very soluble 
in the cell. Hence the higher fatty acids are rapidly absorbed 
by the cell, and the sea-water is practically free from acid. 
Hence it is no longer necessary to transfer the eggs into sea- 
water free from acid. It can be shown that membrane forma- 
tion is prevented by hydrogen ions because no membrane for- 
mation occurs if the eggs are placed, after exposure to butyric 
acid, in sea-water to which some mineral acid has been added. 
For if sea-urchin eggs are transferred after treatment with 
butyric acid to 50 c.c. of sea-water+1.5 or 2 c.c. of N/10 HCl, 
as a rule membrane formation no longer takes place. We 
have mentioned above that the eggs form membranes while in 
1 Loeb, Untersuchungen, p. 343; University of California Publications, Physiol- 
ogy, II, 1905. 
2B. Godlewski, Archiv f. Entwicklungsmechanik, XXVI, 278, 1908. 
