308
Β. Μ. SHAFFER
ability to move over one another's surfaces, and that it creates regional
differences in substratum, which in turn greatly affect the speed of cells
transplanted from one region to another. The regional difference at the
anterior end also produces a change in the shape of the cells there. If it is
correct that such a cell becomes flattened from front to back (Vol. 2, p.
162), this will increase the amount of surface that has to be made in
advancing a given distance, and therefore the work that has to be done
on it to maintain the same velocity.
Despite its regional differences, a slug moves as a whole : short-term
fluctuations
excepted, its back end travels at about the same rate as its
front; but as amputation shows (Francis, 1959), the back does not drag
and hardly pushes. It therefore appears that all the firmly adherent
cells can regulate their motive-energy output to keep up with one
another; but what speed do they keep up with? The tip is obviously in
the best position to set the pace, and a number of experiments seem to
show that it does so.
The basic observations on the tip's role were made by Raper (1940,
1941) in his pioneering study of slug organization. Of these the simplest
and most remarkable has remained unexplained : if the tip is cut off and
not replaced, a slug stops migrating. Generally amputation of 1/20 of the
slug is effective, and occasionally of as little as 1/100. The decapitated
body forms a spherical aggregate at the site of injury. Only later does
this develop a new nipple and transform itself into a normal grex again.
Raper (1941) supposed that a group of amoebae constituted a definite
centre controlling migration. The fact that all the cells in a slug before
it has been decapitated move forward under their own power even
slightly faster than does the isolated tip afterwards, and that all the
cells in the decapitated section continue moving in the same direction as
before till they reach the site of injury (Raper, 1940), suggests to us that
the critical factor that is removed is not one that primarily makes the
cells advance but one that disciplines them into remaining in a cylindrical
mass. This is confirmed by Francis's (1959) observation that the ring
formed by joining together the ends of a decapitated slug will rotate.
Here all the cells could have been kept in order simply by following the
cells ahead of them. What then is the factor that preserves the integrity
of a tip? Obviously it may be related to the accumulation there of a cell
type differing in its surface properties, in ability to move over other
cells, and presumably in adhesiveness, though the change in the shape
of the tip cells, which has been attributed to increased adhesiveness
(Vol. 2, p. 169), must, one supposes, be irrelevant, as it is not found in
young slugs (Bonner, 1944, 1950).
Another disciplinary factor worth considering is the slime sheath. The
only measurements of its mechanical properties are those of Francis
Β. Μ. SHAFFER
ability to move over one another's surfaces, and that it creates regional
differences in substratum, which in turn greatly affect the speed of cells
transplanted from one region to another. The regional difference at the
anterior end also produces a change in the shape of the cells there. If it is
correct that such a cell becomes flattened from front to back (Vol. 2, p.
162), this will increase the amount of surface that has to be made in
advancing a given distance, and therefore the work that has to be done
on it to maintain the same velocity.
Despite its regional differences, a slug moves as a whole : short-term
fluctuations
excepted, its back end travels at about the same rate as its
front; but as amputation shows (Francis, 1959), the back does not drag
and hardly pushes. It therefore appears that all the firmly adherent
cells can regulate their motive-energy output to keep up with one
another; but what speed do they keep up with? The tip is obviously in
the best position to set the pace, and a number of experiments seem to
show that it does so.
The basic observations on the tip's role were made by Raper (1940,
1941) in his pioneering study of slug organization. Of these the simplest
and most remarkable has remained unexplained : if the tip is cut off and
not replaced, a slug stops migrating. Generally amputation of 1/20 of the
slug is effective, and occasionally of as little as 1/100. The decapitated
body forms a spherical aggregate at the site of injury. Only later does
this develop a new nipple and transform itself into a normal grex again.
Raper (1941) supposed that a group of amoebae constituted a definite
centre controlling migration. The fact that all the cells in a slug before
it has been decapitated move forward under their own power even
slightly faster than does the isolated tip afterwards, and that all the
cells in the decapitated section continue moving in the same direction as
before till they reach the site of injury (Raper, 1940), suggests to us that
the critical factor that is removed is not one that primarily makes the
cells advance but one that disciplines them into remaining in a cylindrical
mass. This is confirmed by Francis's (1959) observation that the ring
formed by joining together the ends of a decapitated slug will rotate.
Here all the cells could have been kept in order simply by following the
cells ahead of them. What then is the factor that preserves the integrity
of a tip? Obviously it may be related to the accumulation there of a cell
type differing in its surface properties, in ability to move over other
cells, and presumably in adhesiveness, though the change in the shape
of the tip cells, which has been attributed to increased adhesiveness
(Vol. 2, p. 169), must, one supposes, be irrelevant, as it is not found in
young slugs (Bonner, 1944, 1950).
Another disciplinary factor worth considering is the slime sheath. The
only measurements of its mechanical properties are those of Francis