310
Β. Μ. SHAFFER
had been selected for further advance, much as in the original development of the grex. The purely mechanical role of a surface membrane
could be tested to some extent by seeing whether a decapitated slug
would continue to migrate without interruption into a short glass tube
of the same diameter, but the chemical nature of the membrane might
possibly be decisive in regulating cell movement. The importance of the
slime sheath itself in promoting migration could be examined if it
proved possible, by using a temporary or a permanent ligature, to
remove the tip without severing the slime sheath.
We may now try to assess the part the tip plays in determining the
speed of the whole slug. Francis (1959) found that if a slug had its tip
replaced by the tip of a smaller and therefore slower slug, it was
retarded. Possibly the cells of the rear section simply experienced
difficulty in entering a slime sheath that had been laid down on too small
a scale for them. However, if a slug tip was replaced by a larger tip, the
composite body migrated faster than a normal slug of the same volume.
This agrees with Bonner's conclusion that the anterior slug cells are the
faster ones ; but confronted with the seemingly awkward result that a
severed tip actually moves more slowly than its parent slug, it is
perhaps somewhat of a relief that Bonner's observations can be
interpreted otherwise (Vol. 2, p. 168). In any case, even if the tip cells
do set the pace, we cannot use such regional differences in behaviour to
explain the way in which velocity varies with slug size, because the
relative volumes of the anterior and posterior regions are virtually
independent of absolute size (Bonner, 1957).
Closer examination of the data of Bonner et al. (1953) reveals that in
fact, over the whole range studied, velocity was linearly related to slug
volume rather than to any other measure of slug size. It seems difficult
to explain this relationship completely in terms of a surface resistance of
any kind, if motive power per unit mass is constant. And we might
arrive at the interesting conclusion that in the internal slug milieu, some
factor controlling power output increased with slug volume, or in other
words, that the cells stimulated one another in some cumulative way.
But there is another possible explanation. These data were obtained
from slugs studied individually over a period of several days. During
this time their speed dropped progressively, and their volume was
greatly reduced as a result not only of the loss of individual cells from
the rear but also of the expenditure of energy without taking in any
food. Thus the major or even exclusive cause of the relationship between
volume and speed may have been simply that the slugs got tired. In fact,
if the initial velocities observed are plotted against initial volumes, no
strong trend is revealed, though it may be that all the slugs were at first
so big that their speed was maximal.
Β. Μ. SHAFFER
had been selected for further advance, much as in the original development of the grex. The purely mechanical role of a surface membrane
could be tested to some extent by seeing whether a decapitated slug
would continue to migrate without interruption into a short glass tube
of the same diameter, but the chemical nature of the membrane might
possibly be decisive in regulating cell movement. The importance of the
slime sheath itself in promoting migration could be examined if it
proved possible, by using a temporary or a permanent ligature, to
remove the tip without severing the slime sheath.
We may now try to assess the part the tip plays in determining the
speed of the whole slug. Francis (1959) found that if a slug had its tip
replaced by the tip of a smaller and therefore slower slug, it was
retarded. Possibly the cells of the rear section simply experienced
difficulty in entering a slime sheath that had been laid down on too small
a scale for them. However, if a slug tip was replaced by a larger tip, the
composite body migrated faster than a normal slug of the same volume.
This agrees with Bonner's conclusion that the anterior slug cells are the
faster ones ; but confronted with the seemingly awkward result that a
severed tip actually moves more slowly than its parent slug, it is
perhaps somewhat of a relief that Bonner's observations can be
interpreted otherwise (Vol. 2, p. 168). In any case, even if the tip cells
do set the pace, we cannot use such regional differences in behaviour to
explain the way in which velocity varies with slug size, because the
relative volumes of the anterior and posterior regions are virtually
independent of absolute size (Bonner, 1957).
Closer examination of the data of Bonner et al. (1953) reveals that in
fact, over the whole range studied, velocity was linearly related to slug
volume rather than to any other measure of slug size. It seems difficult
to explain this relationship completely in terms of a surface resistance of
any kind, if motive power per unit mass is constant. And we might
arrive at the interesting conclusion that in the internal slug milieu, some
factor controlling power output increased with slug volume, or in other
words, that the cells stimulated one another in some cumulative way.
But there is another possible explanation. These data were obtained
from slugs studied individually over a period of several days. During
this time their speed dropped progressively, and their volume was
greatly reduced as a result not only of the loss of individual cells from
the rear but also of the expenditure of energy without taking in any
food. Thus the major or even exclusive cause of the relationship between
volume and speed may have been simply that the slugs got tired. In fact,
if the initial velocities observed are plotted against initial volumes, no
strong trend is revealed, though it may be that all the slugs were at first
so big that their speed was maximal.