LOCALIZATION IN THE DEVELOPING FUCUS EGG
319
many other cells (Dainty, 1962; Davson, 1959). Use of these values, then,
yields an estimate of 3 to 30 ^ amp/cm
2 for the current density needed to
determine on the basis of both types of experiment. Since the estimated
self-generated current density is more than 6 n amp/cm
2 , these figures
again suggest a role for the current in effecting determination, if the
imposed gradients really act by driving currents through the egg. But
this is far from clear. Empirically, there is evidence of antagonistic
effects; theoretically it is plain that these vectors may in some circumstances act upon components external to the plasma membrane.
Gross differences appear as the embryo elongates and then divides
into rhizoidal and thallus cells. These are poorly characterized though
there is evidence that ribonucleic acid (RNA) (Nakazawa, 1966), as well
as Golgi apparatus and (fucoidin-bearing?) wall precursor vesicles (Fig.
18) all accumulate toward the rhizoidal tip. The credibility of this
limited evidence is strengthened by similar observations upon other tipgenerated structures such as pollen tubes (Rosen et al., 1964) and Char a
rhizoids (Sievers, 1967). This comparative evidence also suggests that
so-called microvesicles (structures similar in appearance to synaptic
vesicles) may also accumulate at the rhizoid tip.
The question arises as to whether the current helps cause this differentiation. It might bring about differentiation (as well as the earlier
determination) in two essentially different ways.
First, it might act electrophoretically; for in traversing the cytoplasm
it will generate a field which may significantly localize negatively
charged macromolecules or particles toward the growth point (or, if there
are any, positively charged ones at the antipode). On the basis of a
measured cytoplasmic resistivity of 200 ohm-cm, and the inferred current
densities, I estimate the cytoplasmic field during the day after germination to be the order of 10
2 volt/cm. The persistence of stratification in
the centrifuged Fucus furcatus or Pelvetia fastigiata egg during germination indicates that mixing through streaming does not occur (Whitaker,
1940b; Lowrance and Whitaker, 1940). However, an inevitable limitation, the only calculable limitation, and quite possibly the actual limitation upon electrophoretic segregation is the leveling action of diffusion.
At the equilibrium between electrophoresis and backdiffusion, it can be
shown that the gradient, G, of a substance (in fractional change per
centimeter) is given by the field strength, E, in volts per centimeter
multiplied by the ratio of electrophoretic mobility to diffusion constant,
m/D, i.e.,
O = (m/D)E
(1)
319
many other cells (Dainty, 1962; Davson, 1959). Use of these values, then,
yields an estimate of 3 to 30 ^ amp/cm
2 for the current density needed to
determine on the basis of both types of experiment. Since the estimated
self-generated current density is more than 6 n amp/cm
2 , these figures
again suggest a role for the current in effecting determination, if the
imposed gradients really act by driving currents through the egg. But
this is far from clear. Empirically, there is evidence of antagonistic
effects; theoretically it is plain that these vectors may in some circumstances act upon components external to the plasma membrane.
Gross differences appear as the embryo elongates and then divides
into rhizoidal and thallus cells. These are poorly characterized though
there is evidence that ribonucleic acid (RNA) (Nakazawa, 1966), as well
as Golgi apparatus and (fucoidin-bearing?) wall precursor vesicles (Fig.
18) all accumulate toward the rhizoidal tip. The credibility of this
limited evidence is strengthened by similar observations upon other tipgenerated structures such as pollen tubes (Rosen et al., 1964) and Char a
rhizoids (Sievers, 1967). This comparative evidence also suggests that
so-called microvesicles (structures similar in appearance to synaptic
vesicles) may also accumulate at the rhizoid tip.
The question arises as to whether the current helps cause this differentiation. It might bring about differentiation (as well as the earlier
determination) in two essentially different ways.
First, it might act electrophoretically; for in traversing the cytoplasm
it will generate a field which may significantly localize negatively
charged macromolecules or particles toward the growth point (or, if there
are any, positively charged ones at the antipode). On the basis of a
measured cytoplasmic resistivity of 200 ohm-cm, and the inferred current
densities, I estimate the cytoplasmic field during the day after germination to be the order of 10
2 volt/cm. The persistence of stratification in
the centrifuged Fucus furcatus or Pelvetia fastigiata egg during germination indicates that mixing through streaming does not occur (Whitaker,
1940b; Lowrance and Whitaker, 1940). However, an inevitable limitation, the only calculable limitation, and quite possibly the actual limitation upon electrophoretic segregation is the leveling action of diffusion.
At the equilibrium between electrophoresis and backdiffusion, it can be
shown that the gradient, G, of a substance (in fractional change per
centimeter) is given by the field strength, E, in volts per centimeter
multiplied by the ratio of electrophoretic mobility to diffusion constant,
m/D, i.e.,
O = (m/D)E
(1)