168
B. M. SHAFFER
selves within the files according to their height when they first set out,
and subsequently to rearrange themselves whenever one of their number becomes detectably taller or shorter than his neighbours, and whenever they are deranged by an act of God—in other words, if a cell's
position depends not on an innate velocity, but on something of much
greater morphogenetic interest, namely, its reactions to its neighbours.
On this view, anterior cells are faster than posterior ones only when surrounded by them—a conclusion supported by Samuel's (1961) discovery,
made in Bonner's laboratory, that on agar the mean speeds of separate
cells isolated from the front and from the back of slugs are not significantly different.
No visible characteristic corresponding to 'height' in the foregoing
analogy has yet been identified. Bonner at one time believed that the
fastest cells were also the largest, for he found a small but significant
difference in the average size of the anterior and posterior cells, though
the ranges overlapped considerably (Bonner, 1959b); but recently he
worked out that in D. discoideum only a tenth of the anterior cells
undergo mitosis, compared with four-fifths of the posterior ones, and
has offered this as an alternative explanation (Bonner, 1960). This
seems the more probable, as the size differences and most of the cell
divisions are not found until some time after the major sorting has
occurred (Bonner and Frascella, 1952; Bonner, Chiquoine and Kolderie,
1955; Bonner and Adams, 1958). Moreover, Samuel (1961), by comparing cells before and after division, has convincingly shown that the
speed of separate cells on agar is not related to their size. It seems likely
that the relevant gradient along a grex is in a cell's ability to displace
other cells from contact with one another, and thus that it is related to
adhesiveness.
Each of Bonner's strains, carried by mass transfer of spores, contained
a number of genotypes. These, of course, were able to fruit together, but
they were dissimilar enough to distribute themselves differentially
within a grex (Bonner, 1959b), and extremely dissimilar when cloned
(Filosa, 1958). This raises the question as to whether they were all sufficiently alike, and sufficiently unlike all those of other strains, for the
separation of some strain mixtures really to have been as complete as it
appeared to be (Bonner and Adams, 1958).
When the cells of other organisms are mixed up, they may segregate
in accordance with their original position in the body (Roundabush,
1933; Townes and Holtfreter, 1955; Moscona, 1957), and sometimes
also with their species of origin (Galtsoff, 1925). Bonner (1959a) believes
that in slime moulds he has been able to demonstrate only the second
kind of regrouping, though to me it seems that he has shown both. In
any case, his many observations have revealed a remarkable and un-
B. M. SHAFFER
selves within the files according to their height when they first set out,
and subsequently to rearrange themselves whenever one of their number becomes detectably taller or shorter than his neighbours, and whenever they are deranged by an act of God—in other words, if a cell's
position depends not on an innate velocity, but on something of much
greater morphogenetic interest, namely, its reactions to its neighbours.
On this view, anterior cells are faster than posterior ones only when surrounded by them—a conclusion supported by Samuel's (1961) discovery,
made in Bonner's laboratory, that on agar the mean speeds of separate
cells isolated from the front and from the back of slugs are not significantly different.
No visible characteristic corresponding to 'height' in the foregoing
analogy has yet been identified. Bonner at one time believed that the
fastest cells were also the largest, for he found a small but significant
difference in the average size of the anterior and posterior cells, though
the ranges overlapped considerably (Bonner, 1959b); but recently he
worked out that in D. discoideum only a tenth of the anterior cells
undergo mitosis, compared with four-fifths of the posterior ones, and
has offered this as an alternative explanation (Bonner, 1960). This
seems the more probable, as the size differences and most of the cell
divisions are not found until some time after the major sorting has
occurred (Bonner and Frascella, 1952; Bonner, Chiquoine and Kolderie,
1955; Bonner and Adams, 1958). Moreover, Samuel (1961), by comparing cells before and after division, has convincingly shown that the
speed of separate cells on agar is not related to their size. It seems likely
that the relevant gradient along a grex is in a cell's ability to displace
other cells from contact with one another, and thus that it is related to
adhesiveness.
Each of Bonner's strains, carried by mass transfer of spores, contained
a number of genotypes. These, of course, were able to fruit together, but
they were dissimilar enough to distribute themselves differentially
within a grex (Bonner, 1959b), and extremely dissimilar when cloned
(Filosa, 1958). This raises the question as to whether they were all sufficiently alike, and sufficiently unlike all those of other strains, for the
separation of some strain mixtures really to have been as complete as it
appeared to be (Bonner and Adams, 1958).
When the cells of other organisms are mixed up, they may segregate
in accordance with their original position in the body (Roundabush,
1933; Townes and Holtfreter, 1955; Moscona, 1957), and sometimes
also with their species of origin (Galtsoff, 1925). Bonner (1959a) believes
that in slime moulds he has been able to demonstrate only the second
kind of regrouping, though to me it seems that he has shown both. In
any case, his many observations have revealed a remarkable and un-