THE ACRASINA
309
(1962), who has determined both the breaking strength of the sheath
over different parts of the slug and its extensibility after it has been
vacated. He has calculated that the cells cannot exert sufficient force to
expand the sheath to the extent they do if it is really as strong as it
appears to be at the tip. He has therefore concluded that the apical slime
sheath is a solution of low surface tension, and that it solidifies, perhaps
through compression, shearing, or ageing, only as the slug advances
inside it, or when it is being examined. If this view is correct, one would
expect slime to flow immediately over the cut surface of a slug sectioned
near its front end; and one could hardly postulate it as the factor
involved in maintaining migration in an intact slug. However, if the
slime were initially a solution of low surface tension, the tip of a slug
migrating over a water surface would presumably be denuded of slime
whenever it touched the water. If so, it seems improbable that the
naked tip would be able to raise itself off the surface again, or that the
slug could leave behind it the well-defined slime sheath that it does. It
therefore seems likely that the slime sheath is present as a definite
membrane over the extreme tip. How can this be squared with the
calculations showing the apparent inadequacy of the cells' power to
expand such a structure? The answer probably is that the cells make it
rather than break it, and the addition of new material greatly alters the
mechanics of extension.
(In Vol. 2, p. 177, it was stated that it was the sheath's fluidity that
made it contract at the rear of culminating grex. More accurately, it is
its deformability. But the argument advanced there is not affected.
Slime sheath payed out from the rear of a floating slug and lying
freely on the water surface does not markedly contract in length or
diameter, and thus cannot have been stretched elastically while
enveloping the slug. Similarly, the upper and side surfaces do not contract
elastically to the width of the lower surface as the slug vacates the
sheath, and thus they are thrown into folds, visible by phase microscopy,
when they collapse onto it (Shaffer, 1962). When the empty sheath is
lifted off the substratum, it is changed by surface forces from a ribbon
into a thread, except where it is kept extended by rigid support. It is
only these forces that can draw together the multiple stalks of a Ζ).
polycephalum fruiting body.)
If there is indeed a slime membrane over the tip of a grex, perhaps
secreted by the surface cells, it may take some time for it to be reconstituted over the cut surface of a decapitated slug; and meanwhile the
leading cells, trying to crawl past one another, may have advanced on an
expanding front and so produced a spherical tip. Then, even after the
membrane had been re-formed, the sphere might continue to enlarge by
absorbing the remaining cells until some restricted part of the surface
309
(1962), who has determined both the breaking strength of the sheath
over different parts of the slug and its extensibility after it has been
vacated. He has calculated that the cells cannot exert sufficient force to
expand the sheath to the extent they do if it is really as strong as it
appears to be at the tip. He has therefore concluded that the apical slime
sheath is a solution of low surface tension, and that it solidifies, perhaps
through compression, shearing, or ageing, only as the slug advances
inside it, or when it is being examined. If this view is correct, one would
expect slime to flow immediately over the cut surface of a slug sectioned
near its front end; and one could hardly postulate it as the factor
involved in maintaining migration in an intact slug. However, if the
slime were initially a solution of low surface tension, the tip of a slug
migrating over a water surface would presumably be denuded of slime
whenever it touched the water. If so, it seems improbable that the
naked tip would be able to raise itself off the surface again, or that the
slug could leave behind it the well-defined slime sheath that it does. It
therefore seems likely that the slime sheath is present as a definite
membrane over the extreme tip. How can this be squared with the
calculations showing the apparent inadequacy of the cells' power to
expand such a structure? The answer probably is that the cells make it
rather than break it, and the addition of new material greatly alters the
mechanics of extension.
(In Vol. 2, p. 177, it was stated that it was the sheath's fluidity that
made it contract at the rear of culminating grex. More accurately, it is
its deformability. But the argument advanced there is not affected.
Slime sheath payed out from the rear of a floating slug and lying
freely on the water surface does not markedly contract in length or
diameter, and thus cannot have been stretched elastically while
enveloping the slug. Similarly, the upper and side surfaces do not contract
elastically to the width of the lower surface as the slug vacates the
sheath, and thus they are thrown into folds, visible by phase microscopy,
when they collapse onto it (Shaffer, 1962). When the empty sheath is
lifted off the substratum, it is changed by surface forces from a ribbon
into a thread, except where it is kept extended by rigid support. It is
only these forces that can draw together the multiple stalks of a Ζ).
polycephalum fruiting body.)
If there is indeed a slime membrane over the tip of a grex, perhaps
secreted by the surface cells, it may take some time for it to be reconstituted over the cut surface of a decapitated slug; and meanwhile the
leading cells, trying to crawl past one another, may have advanced on an
expanding front and so produced a spherical tip. Then, even after the
membrane had been re-formed, the sphere might continue to enlarge by
absorbing the remaining cells until some restricted part of the surface