164
B. M. S H A F F E R
Curtis (1960) has ascribed the segregation of different cell types to
specificity in the rate at which their surfaces undergo the same nonspecific progressive change. It is not clear to what extent this theory
can be applied to amoeboid cells, but it scarcely seems possible to explain
the specificities discovered in these slime-mould experiments by arranging all the strains in a single temporal series, particularly since it is
reasonable to suppose that nowhere near the maximum number of
existing strain specificities has yet been discovered. It is true the cells of
host and graft were in slightly different stages, but there was no abrupt
discontinuity; moreover, very similar results were obtained with lying
grex, and one of the universal leaders was followed by undifferentiated
cells of the other five strains right up to the moment when it sporulated.
It was particularly striking when the back part of one grex would
follow the front part of one of another strain, but the back of the second
would not follow the front of the first; for as the grex were divided in
arbitrary positions, the cells on either side of the cuts cannot have been
in consistently different states. If adhesion is unspecific, we must then
presumably suppose that there are qualitative differences in the contact
induction of cytoplasmic outflow. Sorting is further considered in
Section V, A, 2.
When a Dictyostelium grex begins to form stalk, the anterior cells lay
down an internal cylindrical sheath. The cells within the sheath become
the first vacuolated stalk cells; the 'transverse' ones outside it extend the sheath as they move forward, are trapped inside it when they
reach the apex, and become additional stalk cells. This reverse fountain
movement, as Bonner has called it, is geometrically an invagination.
But it is worth stressing that the cells at the apex are still moving forward when they are invaginated; they are simply overtaken by cells
further out from the axis. The stalk sheath as laid down is wider and
looser than it ultimately becomes (Brefeld, 1884; Raper and Fennell,
1952). As it grows, it resembles, when erect, a hollow-stemmed wine
glass (Figs. 17 and 18) of which the bowl is being continuously built
upwards at the rim, thickened at the side, and converted into stem at
the bottom. This change in shape must force the cells within the bowl
upwards; and indeed, from measurements on Raper and Fennell's
excellent photographs, it appears that from this cause alone during
much of culmination, cells that have reached the apex must continue to
rise roughly four times the height of the bowl before its bottom catches
up with them. In addition, the stalk cells increase in volume about five
times during vacuolation (Raper and Fennell, 1952), and this too must
push the cell upwards within the sheath. It is true that in D. discoideum
the leading cells do move downwards at the beginning of culmination
before the stalk initial has made contact with the substratum (Bonner,
B. M. S H A F F E R
Curtis (1960) has ascribed the segregation of different cell types to
specificity in the rate at which their surfaces undergo the same nonspecific progressive change. It is not clear to what extent this theory
can be applied to amoeboid cells, but it scarcely seems possible to explain
the specificities discovered in these slime-mould experiments by arranging all the strains in a single temporal series, particularly since it is
reasonable to suppose that nowhere near the maximum number of
existing strain specificities has yet been discovered. It is true the cells of
host and graft were in slightly different stages, but there was no abrupt
discontinuity; moreover, very similar results were obtained with lying
grex, and one of the universal leaders was followed by undifferentiated
cells of the other five strains right up to the moment when it sporulated.
It was particularly striking when the back part of one grex would
follow the front part of one of another strain, but the back of the second
would not follow the front of the first; for as the grex were divided in
arbitrary positions, the cells on either side of the cuts cannot have been
in consistently different states. If adhesion is unspecific, we must then
presumably suppose that there are qualitative differences in the contact
induction of cytoplasmic outflow. Sorting is further considered in
Section V, A, 2.
When a Dictyostelium grex begins to form stalk, the anterior cells lay
down an internal cylindrical sheath. The cells within the sheath become
the first vacuolated stalk cells; the 'transverse' ones outside it extend the sheath as they move forward, are trapped inside it when they
reach the apex, and become additional stalk cells. This reverse fountain
movement, as Bonner has called it, is geometrically an invagination.
But it is worth stressing that the cells at the apex are still moving forward when they are invaginated; they are simply overtaken by cells
further out from the axis. The stalk sheath as laid down is wider and
looser than it ultimately becomes (Brefeld, 1884; Raper and Fennell,
1952). As it grows, it resembles, when erect, a hollow-stemmed wine
glass (Figs. 17 and 18) of which the bowl is being continuously built
upwards at the rim, thickened at the side, and converted into stem at
the bottom. This change in shape must force the cells within the bowl
upwards; and indeed, from measurements on Raper and Fennell's
excellent photographs, it appears that from this cause alone during
much of culmination, cells that have reached the apex must continue to
rise roughly four times the height of the bowl before its bottom catches
up with them. In addition, the stalk cells increase in volume about five
times during vacuolation (Raper and Fennell, 1952), and this too must
push the cell upwards within the sheath. It is true that in D. discoideum
the leading cells do move downwards at the beginning of culmination
before the stalk initial has made contact with the substratum (Bonner,