THE ACRASINA
155
inactivation at a more neutral p H was attributed to an enzyme converting the second inhibitor into the first (Sussman, Lee, and Kerr, 1956;
R. R. Sussman et al. 9 1958).
Another line of attack began with the discovery that pregnancy urine
and various pure steroids at very low concentrations were active in
the agar-block test (Wright and Anderson, 1958). This led to the isolation in strong acid of a remarkable amount of a weakly active steroid
from D. discoideum, identified as Zl
22 -stigmasten-3/3-ol (Heftmann et al.,
1959). The authors suggested that acrasin itself—apparently meaning
the attractor—might be a conjugate or act synergistically. Interestingly
enough, steroids have been found to affect the surface of the solitary
amoeba Naegleria, regulating the change to the flagellate form (Pearson,
quoted in Willmer, 1960).
It is doubtful whether stigmastenol can be readily inactivated by a
culture extract (Wright, privately), perhaps simply because it is not the
natural attractor. But it must be admitted that the whole question of
inactivation remains obscure, for despite its theoretical advantages, the
pattern of aggregation is not seriously altered at pH values at least as
low as 4 (Hirschberg and Rusch, 1950; Shaffer, 1957c), although the
inactivating enzyme itself is said to be inactive at a pH at least as high
as 3-5 (Sussman, Lee, and Kerr, 1956). Moreover, Pfützner-Eckert (1950)
reported that sensitive cells of mucoroides would stream towards a
block of agar that had underlain a centre, and form their own centre on
top of it; this suggests that the attractor is stable. But others (Bonner,
1959a; Shaffer, 1953a) failed to detect attraction in similar experiments (the criterion was rather gross, and it is quite likely that careful
microscopy would have revealed a transient phase of pseudopodial
orientation). Pfützner-Eckert also reported that if the centre of an
aggregation was displaced as much as 1500 μ, a distance several times
the maximum direct range found by anyone else, the streams would be
re-attracted to it, and would follow the actual path along which it had
been pushed over the agar, even if this was a zig-zag. These observations
could not be confirmed in my work (Shaffer, 1957b). Perhaps PfütznerEckert used a strain of vastly greater sensitivity, but this is not supported by the streams' interactions. It is clear that she regarded hemispherical centres as the sole source of attractor during aggregation, and
thus did not allow for the activity of any cells left behind on the path
of a displaced centre, some of which are clearly visible in one drawing.
Thus she may have left a few secreting cells on her agar blocks as well.
And as the blocks were simply laid on top of the layer of test cells,
another explanation could be that the agar contained some relatively
stable substance that stimulated the adjacent cells to form centres.
It remain'? possible that, in any one species, the attractor, the in-
155
inactivation at a more neutral p H was attributed to an enzyme converting the second inhibitor into the first (Sussman, Lee, and Kerr, 1956;
R. R. Sussman et al. 9 1958).
Another line of attack began with the discovery that pregnancy urine
and various pure steroids at very low concentrations were active in
the agar-block test (Wright and Anderson, 1958). This led to the isolation in strong acid of a remarkable amount of a weakly active steroid
from D. discoideum, identified as Zl
22 -stigmasten-3/3-ol (Heftmann et al.,
1959). The authors suggested that acrasin itself—apparently meaning
the attractor—might be a conjugate or act synergistically. Interestingly
enough, steroids have been found to affect the surface of the solitary
amoeba Naegleria, regulating the change to the flagellate form (Pearson,
quoted in Willmer, 1960).
It is doubtful whether stigmastenol can be readily inactivated by a
culture extract (Wright, privately), perhaps simply because it is not the
natural attractor. But it must be admitted that the whole question of
inactivation remains obscure, for despite its theoretical advantages, the
pattern of aggregation is not seriously altered at pH values at least as
low as 4 (Hirschberg and Rusch, 1950; Shaffer, 1957c), although the
inactivating enzyme itself is said to be inactive at a pH at least as high
as 3-5 (Sussman, Lee, and Kerr, 1956). Moreover, Pfützner-Eckert (1950)
reported that sensitive cells of mucoroides would stream towards a
block of agar that had underlain a centre, and form their own centre on
top of it; this suggests that the attractor is stable. But others (Bonner,
1959a; Shaffer, 1953a) failed to detect attraction in similar experiments (the criterion was rather gross, and it is quite likely that careful
microscopy would have revealed a transient phase of pseudopodial
orientation). Pfützner-Eckert also reported that if the centre of an
aggregation was displaced as much as 1500 μ, a distance several times
the maximum direct range found by anyone else, the streams would be
re-attracted to it, and would follow the actual path along which it had
been pushed over the agar, even if this was a zig-zag. These observations
could not be confirmed in my work (Shaffer, 1957b). Perhaps PfütznerEckert used a strain of vastly greater sensitivity, but this is not supported by the streams' interactions. It is clear that she regarded hemispherical centres as the sole source of attractor during aggregation, and
thus did not allow for the activity of any cells left behind on the path
of a displaced centre, some of which are clearly visible in one drawing.
Thus she may have left a few secreting cells on her agar blocks as well.
And as the blocks were simply laid on top of the layer of test cells,
another explanation could be that the agar contained some relatively
stable substance that stimulated the adjacent cells to form centres.
It remain'? possible that, in any one species, the attractor, the in-