18
CHR. P. RAVEN
it is mutual. Small amounts of CaCl 2 diminish the lethal effects of alkali
ions, but inversely small amounts of alkali chlorides relieve the effects
of lethal concentrations of calcium.
Further consideration of these results has led to the conclusion that a
certain amount of calcium ions in the cortex is necessary for its stability
and physiological integrity. When the eggs are treated with alkali
salts there is an exchange between the calcium ions of the cortex
and the alkali cations. At a certain concentration of the added salt the
calcium loss becomes too large, and mortality occurs ; but even at lower
concentrations the bound alkali ions may influence the properties of the
cortex, and thereby the course of development, in a specific way.
Addition of calcium ions to the salt solution diminishes the harmful
effect of the alkali ions. On the other hand, high concentrations of
calcium in the medium cause a decrease in charge and thereby an
increase in density of the cortex, until the upper limit of the physiological
range is exceeded and mortality occurs. This may be counteracted by
the addition of alkali salts in low concentrations, as a consequence of ion
antagonism which is mainly due to a lowering of the activity of calcium
in the medium by the added anions. In this way the mutual antagonism
can be explained (Raven, 1957 ; Geilenkirchen, 1961).
This view has been further tested by Geilenkirchen (1961) in a
comparative study of the effect of the alkaline earth ions magnesium,
calcium, strontium and barium. The concentrations of MgCl 2 and SrCl 2
provoking 50 per cent death were found to be close to that for CaCl 2 ,
according to the sequence Mg
caused considerable mortality at lower concentrations, while BaCl 2
caused an increasing death-rate up to 100 per cent at still lower concentrations. These second effects of alkaline earth ions may be assumed to
be due to an exchange with calcium in the cell membrane. Mg and Sr are
supposed to a certain extent to be able to replace calcium in its effects on
the stability of the membrane, which explains the shape of their
mortality curves. It is also possible that the effects of these cations are
related to their effectivity in maintaining a tri-complex system in the
cell membrane.
Summarizing : these investigations point to the existence of a surface
layer of the egg consisting of a colloid complex, in which proteins,
phosphatides and probably ribonucleic acid take part, while calcium
ions are essential for its stability and physiological integrity.
In electron-microscopic observations of the Limnaea egg (Elbers,
1959) the only visible differentiation of the egg surface is formed by a
double membrane, consisting of two dark lines separated by a clear zone.
The total thickness of this membrane is about 104 Â, of which the outer
CHR. P. RAVEN
it is mutual. Small amounts of CaCl 2 diminish the lethal effects of alkali
ions, but inversely small amounts of alkali chlorides relieve the effects
of lethal concentrations of calcium.
Further consideration of these results has led to the conclusion that a
certain amount of calcium ions in the cortex is necessary for its stability
and physiological integrity. When the eggs are treated with alkali
salts there is an exchange between the calcium ions of the cortex
and the alkali cations. At a certain concentration of the added salt the
calcium loss becomes too large, and mortality occurs ; but even at lower
concentrations the bound alkali ions may influence the properties of the
cortex, and thereby the course of development, in a specific way.
Addition of calcium ions to the salt solution diminishes the harmful
effect of the alkali ions. On the other hand, high concentrations of
calcium in the medium cause a decrease in charge and thereby an
increase in density of the cortex, until the upper limit of the physiological
range is exceeded and mortality occurs. This may be counteracted by
the addition of alkali salts in low concentrations, as a consequence of ion
antagonism which is mainly due to a lowering of the activity of calcium
in the medium by the added anions. In this way the mutual antagonism
can be explained (Raven, 1957 ; Geilenkirchen, 1961).
This view has been further tested by Geilenkirchen (1961) in a
comparative study of the effect of the alkaline earth ions magnesium,
calcium, strontium and barium. The concentrations of MgCl 2 and SrCl 2
provoking 50 per cent death were found to be close to that for CaCl 2 ,
according to the sequence Mg
caused considerable mortality at lower concentrations, while BaCl 2
caused an increasing death-rate up to 100 per cent at still lower concentrations. These second effects of alkaline earth ions may be assumed to
be due to an exchange with calcium in the cell membrane. Mg and Sr are
supposed to a certain extent to be able to replace calcium in its effects on
the stability of the membrane, which explains the shape of their
mortality curves. It is also possible that the effects of these cations are
related to their effectivity in maintaining a tri-complex system in the
cell membrane.
Summarizing : these investigations point to the existence of a surface
layer of the egg consisting of a colloid complex, in which proteins,
phosphatides and probably ribonucleic acid take part, while calcium
ions are essential for its stability and physiological integrity.
In electron-microscopic observations of the Limnaea egg (Elbers,
1959) the only visible differentiation of the egg surface is formed by a
double membrane, consisting of two dark lines separated by a clear zone.
The total thickness of this membrane is about 104 Â, of which the outer