162
B. M. SHAFFER
description been published of cell shape in the transverse plane; and
here the cells, when arranged in a single layer around the extremely thin
stalk, form sectors of a circle (Figs. 15 and 16). At present we may
regard 'transverse' cells as being flattened from front to back, having
changed not their direction but their shape (Section V, A, 3 and Fig.
19(c)). They are like piles of noncircular coins, their heads all facing the
same way, packed together into a cylindrical stack. The fact that those
of them actually in contact with the outside of the stalk may accumulate polysaccharide in the cytoplasmic region that is next to it (Bonner,
Chiquoine, and Kolderie, 1955), and may be elongated, is not in conflict with this view: men marching in a column are usually oriented
towards its front.
If the grex cells move like stream cells, we must again consider to
what extent they are guided by acrasin. Bonner (1949) examined the
competitive attractiveness of parts of a D. discoideum slug and believed
he could demonstrate that about half the acrasin secreted came from
the apical tenth. Unfortunately, this figure needs revision for a number
of reasons (Shaffer, 1953b); no allowance was made for acrasin inactivation and induced secretion (both unsuspected at the time), nor for differences in the shape and size of the cell masses. Had the interpretation
been valid, the same test could have been used to show that any tenth,
or indeed within limits any other fraction, secreted about half the total
acrasin. The published measurements do not in fact strongly contradict
the statement that the whole surface of the slug commonly secretes
attractor at approximately the same concentration. A less quantitative
method revealed that though release of attractor was progressively
restricted to the anterior end during culmination (Bonner, 1949), no
emission gradient was detectable along any part of the young slugs, or at
least the posterior nine-tenths of most of the older ones (Bonner and
Slifkin, 1949). With the further evidence that the back end of a normal
slug (Bonner, 1949), and perhaps of a D. mucoroides grex (as suggested
by one of Pfützner-Eckert's (1950) illustrations), may sometimes be a
region of higher production; that a slug may be joined head to tail to
form a ring that rotates for some hours (Francis, 1959); and that a
reversed slug tip grafted on to the rear of a slug does not induce host
cells to reorient and follow it (Raper, 1940b); it may be concluded that
there is normally no continuous gradient of attractor along a slug's
axis; that orderly cell movement is maintained in its absence; and that
the maximum difference that may exist between different regions is
insufficient to reverse a cell's direction. Possibly sustained or propagated
differentials are important in establishing the axis of the grex initially.
Contact following, on the other hand, can act in a grex as well as in a
stream. It can account for the persistent polarity of all sections of slugs,
B. M. SHAFFER
description been published of cell shape in the transverse plane; and
here the cells, when arranged in a single layer around the extremely thin
stalk, form sectors of a circle (Figs. 15 and 16). At present we may
regard 'transverse' cells as being flattened from front to back, having
changed not their direction but their shape (Section V, A, 3 and Fig.
19(c)). They are like piles of noncircular coins, their heads all facing the
same way, packed together into a cylindrical stack. The fact that those
of them actually in contact with the outside of the stalk may accumulate polysaccharide in the cytoplasmic region that is next to it (Bonner,
Chiquoine, and Kolderie, 1955), and may be elongated, is not in conflict with this view: men marching in a column are usually oriented
towards its front.
If the grex cells move like stream cells, we must again consider to
what extent they are guided by acrasin. Bonner (1949) examined the
competitive attractiveness of parts of a D. discoideum slug and believed
he could demonstrate that about half the acrasin secreted came from
the apical tenth. Unfortunately, this figure needs revision for a number
of reasons (Shaffer, 1953b); no allowance was made for acrasin inactivation and induced secretion (both unsuspected at the time), nor for differences in the shape and size of the cell masses. Had the interpretation
been valid, the same test could have been used to show that any tenth,
or indeed within limits any other fraction, secreted about half the total
acrasin. The published measurements do not in fact strongly contradict
the statement that the whole surface of the slug commonly secretes
attractor at approximately the same concentration. A less quantitative
method revealed that though release of attractor was progressively
restricted to the anterior end during culmination (Bonner, 1949), no
emission gradient was detectable along any part of the young slugs, or at
least the posterior nine-tenths of most of the older ones (Bonner and
Slifkin, 1949). With the further evidence that the back end of a normal
slug (Bonner, 1949), and perhaps of a D. mucoroides grex (as suggested
by one of Pfützner-Eckert's (1950) illustrations), may sometimes be a
region of higher production; that a slug may be joined head to tail to
form a ring that rotates for some hours (Francis, 1959); and that a
reversed slug tip grafted on to the rear of a slug does not induce host
cells to reorient and follow it (Raper, 1940b); it may be concluded that
there is normally no continuous gradient of attractor along a slug's
axis; that orderly cell movement is maintained in its absence; and that
the maximum difference that may exist between different regions is
insufficient to reverse a cell's direction. Possibly sustained or propagated
differentials are important in establishing the axis of the grex initially.
Contact following, on the other hand, can act in a grex as well as in a
stream. It can account for the persistent polarity of all sections of slugs,