LOCALIZATION IN THE DEVELOPING FUCUS EGG
321
value is most easily appraised by considering spheres in aqueous media.
Judging from bacterial behavior, one may guess that some cell components will be so densely charged as to move at 4 /x/sec per volt per
centimeter in such media (James, 1957). For spheres in water, D =
2.10
1 3
/r (Setlow and Pollard, 1962). So the ratio m/D is 2.10
9
r volt"
1
for such components. Then using Eq. (1) one gets 2.10
7
r cm
-1 for their
equilibrium gradient in a field of 10~
2 volt/cm.
This reaches 10% per egg diameter for particles of the order of 100 A.
Hence electrophoretic segregation is one plausible mode of action for
the current.
Second, the above consideration of the mechanism of current propulsion plainly suggested that some ion other than potassium enters the
growth pole while potassium leaves at the antipode. If this is true, then
the current could directly create qualitative ion gradients and in this
second way cause differentiation.
My minimal estimate of the mean flux entering the rhizoidal region
during the first day of differentiation is 20 ^amp/cm
2 , and this corresponds to a turnover of 0.2 M/hour in a 30-/x long cell. If there arises
a sufficient transembryonic diffusion barrier to the entering ion, then
this turnover will plainly suffice to set up gross ion gradients.
III. Localizing Electrical Currents in Other Systems
A. Self-generated Currents
The finding of a developmental current through the Fucus egg seems
to be a valuable lead toward understanding localization in this system.
There exists a vast literature of potential measurements on other developing systems (Lund, 1947; Flickinger and Blount, 1957; Parkinson and
Banbury, 1966). None of it is informed by the concept of developmental
currents. Yet a question arises: Can one salvage any evidence of such
currents from it?
There are three main requirements for extracting a measure of developmental currents from potential measurements:
1. The system must be developing during the measurement in a known
way.
2. Potential differences must be convertible into current densities.
Hence the resistances between the points probed must be known.
3. The concentration profile in the medium between the electrodes
must be sufficiently symmetrical to avoid confusion by extracellular
concentration gradients.
321
value is most easily appraised by considering spheres in aqueous media.
Judging from bacterial behavior, one may guess that some cell components will be so densely charged as to move at 4 /x/sec per volt per
centimeter in such media (James, 1957). For spheres in water, D =
2.10
1 3
/r (Setlow and Pollard, 1962). So the ratio m/D is 2.10
9
r volt"
1
for such components. Then using Eq. (1) one gets 2.10
7
r cm
-1 for their
equilibrium gradient in a field of 10~
2 volt/cm.
This reaches 10% per egg diameter for particles of the order of 100 A.
Hence electrophoretic segregation is one plausible mode of action for
the current.
Second, the above consideration of the mechanism of current propulsion plainly suggested that some ion other than potassium enters the
growth pole while potassium leaves at the antipode. If this is true, then
the current could directly create qualitative ion gradients and in this
second way cause differentiation.
My minimal estimate of the mean flux entering the rhizoidal region
during the first day of differentiation is 20 ^amp/cm
2 , and this corresponds to a turnover of 0.2 M/hour in a 30-/x long cell. If there arises
a sufficient transembryonic diffusion barrier to the entering ion, then
this turnover will plainly suffice to set up gross ion gradients.
III. Localizing Electrical Currents in Other Systems
A. Self-generated Currents
The finding of a developmental current through the Fucus egg seems
to be a valuable lead toward understanding localization in this system.
There exists a vast literature of potential measurements on other developing systems (Lund, 1947; Flickinger and Blount, 1957; Parkinson and
Banbury, 1966). None of it is informed by the concept of developmental
currents. Yet a question arises: Can one salvage any evidence of such
currents from it?
There are three main requirements for extracting a measure of developmental currents from potential measurements:
1. The system must be developing during the measurement in a known
way.
2. Potential differences must be convertible into current densities.
Hence the resistances between the points probed must be known.
3. The concentration profile in the medium between the electrodes
must be sufficiently symmetrical to avoid confusion by extracellular
concentration gradients.