VITELLINE MEMBRANE AND CORTICAL PARTICLES 265
number of aliphatic and aromatic carboxylic acids is of particular interest. As is well known, Loeb (1913) introduced the method of activating
sea urchin eggs by brief exposure to butyric acid and subsequent transfer
into sea water of normal hydrogen-ion concentration. The elaborate
experiments carried out by Isaka and Aikawa (loc. cit.) confirmed the
hypothesis of Loeb that carboxylic acids enter the eggs as neutral molecules. The authors exposed the unfertilized eggs to the acid in sea water
at a pH corresponding to the pK value of the acids. After return to normal medium, the acids become fully ionized and were then able to break
up hydrogen bonds.
As pointed out by Isaka and Aikawa (1963a,b), single hydrogen
bonds have a rather low energy, but if a great number of them are
present, the energy may be sufficient to keep the vitelline membrane
firmly attached to the egg surface.
Carboxylic acids presumably function only as neutral molecules because they need to penetrate through one or more negatively charged
membranes. Those to be considered are the vitelline and the plasma
membrane. Available evidence points to the plasma membrane as the
main ion barrier. If the vitelline membrane had a strong negative charge,
it would repel anions. The carboxylic acid, however, also penetrates
through the plasma membrane because at optimal butyric acid concentration and time of exposure the cortical particles are opened. In this
way a complete formation of a membrane occurs that has the same composite nature as a fertilization membrane.
The effects produced by acid treatment are thus complex and include
more than the breakage of hydrogen bonds connecting the vitelline and
the plasma membranes. Even if the vitelline membrane elevates before
the cortical particles open, this does not prove a causal relationship (see
Section II,C). In only one case did a mere separation of the vitelline
membrane from the egg surface occur in the experiments by Isaka and
Aikawa (loc. cit.), viz., in eggs of Hemicentrotus,
subjected to guanidine
at pH 9.0. In this case, the substance probably did not penetrate through
the plasma membrane but acted merely on the bonds attaching the vitelline membrane to the cytoplasmic surface.
The question arises as to whether the activation by acids reflects a
normal process. In fact, a transient acid formation (Runnström, 1933;
Borei, 1934; Laser and Rothschild, 1939; Allen et al. y 1958; Aketa,
1961a,b, 1963; Mehl and Swann, 1961; Ohnishi and Sugiyama, 1963)
occurs very early after insemination. The titrimetric data presented by
Ohnishi and Sugiyama (1963) obtained with eggs of
Hemicentrotus
(temperature 20°C) indicate that the acid formation starts immediately
after the effective sperm attachment. When the cortical particles open,
number of aliphatic and aromatic carboxylic acids is of particular interest. As is well known, Loeb (1913) introduced the method of activating
sea urchin eggs by brief exposure to butyric acid and subsequent transfer
into sea water of normal hydrogen-ion concentration. The elaborate
experiments carried out by Isaka and Aikawa (loc. cit.) confirmed the
hypothesis of Loeb that carboxylic acids enter the eggs as neutral molecules. The authors exposed the unfertilized eggs to the acid in sea water
at a pH corresponding to the pK value of the acids. After return to normal medium, the acids become fully ionized and were then able to break
up hydrogen bonds.
As pointed out by Isaka and Aikawa (1963a,b), single hydrogen
bonds have a rather low energy, but if a great number of them are
present, the energy may be sufficient to keep the vitelline membrane
firmly attached to the egg surface.
Carboxylic acids presumably function only as neutral molecules because they need to penetrate through one or more negatively charged
membranes. Those to be considered are the vitelline and the plasma
membrane. Available evidence points to the plasma membrane as the
main ion barrier. If the vitelline membrane had a strong negative charge,
it would repel anions. The carboxylic acid, however, also penetrates
through the plasma membrane because at optimal butyric acid concentration and time of exposure the cortical particles are opened. In this
way a complete formation of a membrane occurs that has the same composite nature as a fertilization membrane.
The effects produced by acid treatment are thus complex and include
more than the breakage of hydrogen bonds connecting the vitelline and
the plasma membranes. Even if the vitelline membrane elevates before
the cortical particles open, this does not prove a causal relationship (see
Section II,C). In only one case did a mere separation of the vitelline
membrane from the egg surface occur in the experiments by Isaka and
Aikawa (loc. cit.), viz., in eggs of Hemicentrotus,
subjected to guanidine
at pH 9.0. In this case, the substance probably did not penetrate through
the plasma membrane but acted merely on the bonds attaching the vitelline membrane to the cytoplasmic surface.
The question arises as to whether the activation by acids reflects a
normal process. In fact, a transient acid formation (Runnström, 1933;
Borei, 1934; Laser and Rothschild, 1939; Allen et al. y 1958; Aketa,
1961a,b, 1963; Mehl and Swann, 1961; Ohnishi and Sugiyama, 1963)
occurs very early after insemination. The titrimetric data presented by
Ohnishi and Sugiyama (1963) obtained with eggs of
Hemicentrotus
(temperature 20°C) indicate that the acid formation starts immediately
after the effective sperm attachment. When the cortical particles open,