306
Β. Μ. SHAFFER
Dictyostélium the prespore cells do not make a significant contribution to
lifting the grex even at the start of culmination. Evidence that they do
will be presented in Section V, B, 3.
Several of the difficulties of understanding the movement of a grex
disappear if in fact all its cells that have not yet differentiated into stalk
or spores, including those commonly described as transverse, are
actually oriented towards its anterior end (Fig. 19, Vol. 2, p. 170).
We may thus reasonably assume that a grex, whether culminating or
lying, advances by the amoeboid movement of all its cells.
Several studies have now been made of grex velocities in the hope that
they would throw light on how these unitary cell masses are organized.
Bonner and Eldredge (1945) found that the rate of ascent of culminating
D. discoideum grex was proportional to their linear dimensions. They
accounted for this by the principle of similitude—that the ratio of
volume to surface of bodies of the same shape increases with their linear
dimensions. They supposed that the morphogenetic force per unit mass
was independent of load and therefore constant in grex of all sizes, and
that it was opposed by a resistance, tentatively identified as the cohesion
between grex and stalk, proportional to the surface area. Bonner et al.
(1953) reported that D. discoideum slugs too obeyed the principle of
similitude : their velocity increased with size. This was the opposite of
what would have been expected if their movement depended on the
activity of the surface layer of cells alone, and it was thus an important
part of the evidence that the whole cell mass was involved. (Other
evidence is that individual cells can be seen to change their position
(Bonner, 1957), and that if a cell moves as it does in a monolayer, there
is no way for it to carry other cells along passively on its back.) The
precise relation between velocity and size has not yet been adequately
accounted for. Bonner and his co-workers suggested that slug speed
increased because the power available for movement was opposed by
some surface resistance such as friction. However, as the substratum for
an internal cell is the surface of its neighbours, the ratio of volume to
this surface will remain constant whatever the size of the grex. Francis
(1959) found that, with slugs of the same volume, the longer and thinner
they were, and therefore the larger their surface area, the faster they
moved. This led him to point out that the only slug surface likely to
offer a serious resistance was at the anterior end, for here the slime
sheath has to be expanded.
Extensive measurement of the properties of entire slugs, as well
as of fragments, grafts, and components (Francis, 1962), suggested a
quantitative model of a slug pictured essentially as a crowd of cells that
are not on speaking terms, imprisoned in a sack of their own manufacture, shoving with all their might to try to escape from the neck of the
Β. Μ. SHAFFER
Dictyostélium the prespore cells do not make a significant contribution to
lifting the grex even at the start of culmination. Evidence that they do
will be presented in Section V, B, 3.
Several of the difficulties of understanding the movement of a grex
disappear if in fact all its cells that have not yet differentiated into stalk
or spores, including those commonly described as transverse, are
actually oriented towards its anterior end (Fig. 19, Vol. 2, p. 170).
We may thus reasonably assume that a grex, whether culminating or
lying, advances by the amoeboid movement of all its cells.
Several studies have now been made of grex velocities in the hope that
they would throw light on how these unitary cell masses are organized.
Bonner and Eldredge (1945) found that the rate of ascent of culminating
D. discoideum grex was proportional to their linear dimensions. They
accounted for this by the principle of similitude—that the ratio of
volume to surface of bodies of the same shape increases with their linear
dimensions. They supposed that the morphogenetic force per unit mass
was independent of load and therefore constant in grex of all sizes, and
that it was opposed by a resistance, tentatively identified as the cohesion
between grex and stalk, proportional to the surface area. Bonner et al.
(1953) reported that D. discoideum slugs too obeyed the principle of
similitude : their velocity increased with size. This was the opposite of
what would have been expected if their movement depended on the
activity of the surface layer of cells alone, and it was thus an important
part of the evidence that the whole cell mass was involved. (Other
evidence is that individual cells can be seen to change their position
(Bonner, 1957), and that if a cell moves as it does in a monolayer, there
is no way for it to carry other cells along passively on its back.) The
precise relation between velocity and size has not yet been adequately
accounted for. Bonner and his co-workers suggested that slug speed
increased because the power available for movement was opposed by
some surface resistance such as friction. However, as the substratum for
an internal cell is the surface of its neighbours, the ratio of volume to
this surface will remain constant whatever the size of the grex. Francis
(1959) found that, with slugs of the same volume, the longer and thinner
they were, and therefore the larger their surface area, the faster they
moved. This led him to point out that the only slug surface likely to
offer a serious resistance was at the anterior end, for here the slime
sheath has to be expanded.
Extensive measurement of the properties of entire slugs, as well
as of fragments, grafts, and components (Francis, 1962), suggested a
quantitative model of a slug pictured essentially as a crowd of cells that
are not on speaking terms, imprisoned in a sack of their own manufacture, shoving with all their might to try to escape from the neck of the