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B. M. SHAFFER
cells, grown on collodion-covered agar to prevent them dispersing.
The R-cell paradox. Slightly more than 50% of the I-cells (a figure
obtained by simply subtracting the background frequency, as the authors
did for the R-cells) transferred singly to small, newly centrifuged, test
populations induced extra aggregations in them (Ennis and Sussman,
1958b). Individual R-cells had no such effect if transferred immediately
after being centrifuged, but 25%of them did if taken from populations
that had already been lying on agar for 10-12 hours; this rise in the
'initiative capacity' of the R-cells was not related to the presence of
I-cells during ageing. The paradox was that in the absence of I-cells,
the old R-cell populations never aggregated however long they were kept
(Sussman and Ennis, 1959).
Comment. Though to the authors the R-cells' capacity remained far
short of the I-cells', in fact the total inductive potential of the R-cells
in the population would appear to have reached ^1,000 times that of
the I-cells, and moreover was still rising after 12-hours ageing. Other
experiments were held to show that R-cells during the same period of
ageing could be given the power to aggregate by I-cells acting over
'truly fantastic distances'. But if this was so, and yet R-cells had no
effect on one another though coming in actual contact, they cannot have
been secreting the initiating substance; and they must either have been
stimulated to do so by the newly centrifuged cells or have stimulated
them to. In either case, the fact that periods of evocation and competence failed to overlap would have been important only in populations
brought experimentally to a uniform stage of development. In more
natural populations, one would expect to find in close proximity cells
that had just stopped feeding and those that had done so many hours
earlier. Sussman and Ennis concluded: 'the initiative capacity of. . .
R-cells . . . is an experimental artifact bearing no relation to normal
aggregation.' But on this ground alone, we may conclude that the initiative capacity of R-cells is the basis of normal aggregation, and the
demonstration of the indispensability of I-cells the experimental artifact. Furthermore, even if pure, synchronized R-cells were in fact unable to aggregate, as these authors claimed, it might be reasonably
supposed, having regard to the known effects of centrifuging, that the
spontaneous rise in initiative capacity as the R-cells aged was due simply
to a process of recovery, and the need for cells of different ages to interact, and hence the R-cell paradox, to differential destruction of competence and evocating power during preparation.
We may safely conclude that the I-cell is not, as Ennis and Sussman
(1958b) believed, Hhe initiator for slime mold aggregation' (original
italics), not even in one particular strain of one particular species. However, it would hardly be surprising, though perhaps not of great signifi-
B. M. SHAFFER
cells, grown on collodion-covered agar to prevent them dispersing.
The R-cell paradox. Slightly more than 50% of the I-cells (a figure
obtained by simply subtracting the background frequency, as the authors
did for the R-cells) transferred singly to small, newly centrifuged, test
populations induced extra aggregations in them (Ennis and Sussman,
1958b). Individual R-cells had no such effect if transferred immediately
after being centrifuged, but 25%of them did if taken from populations
that had already been lying on agar for 10-12 hours; this rise in the
'initiative capacity' of the R-cells was not related to the presence of
I-cells during ageing. The paradox was that in the absence of I-cells,
the old R-cell populations never aggregated however long they were kept
(Sussman and Ennis, 1959).
Comment. Though to the authors the R-cells' capacity remained far
short of the I-cells', in fact the total inductive potential of the R-cells
in the population would appear to have reached ^1,000 times that of
the I-cells, and moreover was still rising after 12-hours ageing. Other
experiments were held to show that R-cells during the same period of
ageing could be given the power to aggregate by I-cells acting over
'truly fantastic distances'. But if this was so, and yet R-cells had no
effect on one another though coming in actual contact, they cannot have
been secreting the initiating substance; and they must either have been
stimulated to do so by the newly centrifuged cells or have stimulated
them to. In either case, the fact that periods of evocation and competence failed to overlap would have been important only in populations
brought experimentally to a uniform stage of development. In more
natural populations, one would expect to find in close proximity cells
that had just stopped feeding and those that had done so many hours
earlier. Sussman and Ennis concluded: 'the initiative capacity of. . .
R-cells . . . is an experimental artifact bearing no relation to normal
aggregation.' But on this ground alone, we may conclude that the initiative capacity of R-cells is the basis of normal aggregation, and the
demonstration of the indispensability of I-cells the experimental artifact. Furthermore, even if pure, synchronized R-cells were in fact unable to aggregate, as these authors claimed, it might be reasonably
supposed, having regard to the known effects of centrifuging, that the
spontaneous rise in initiative capacity as the R-cells aged was due simply
to a process of recovery, and the need for cells of different ages to interact, and hence the R-cell paradox, to differential destruction of competence and evocating power during preparation.
We may safely conclude that the I-cell is not, as Ennis and Sussman
(1958b) believed, Hhe initiator for slime mold aggregation' (original
italics), not even in one particular strain of one particular species. However, it would hardly be surprising, though perhaps not of great signifi-