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CHARLES S. THORNTON
cap yet did not penetrate through it to affect mesodermal cells in the
inner core of the limb stump. Apical cap formation and accumulation
of blastemal cells were prevented by ultraviolet treatment (Thornton,
1958). Application of ultraviolet light is an unnatural circumstance and
might conceivably interfere with limb metabolism in unknown (and
undetected) ways. Moving the apical cap to an asymmetrical position
seemed to offer the promise of an unequivocal answer to the cap's possible influence on blastemal cell aggregation. Would an acentric cap
induce an acentric blastema beneath it? To test the possibility, the
wound epithelium of the stump tip was induced to move across and, thus,
heal an adjacent skin wound. The apical cap was carried along with the
migrating epidermis, of which it was a part, so that it came to lie at one
side of the limb tip where blastemal cells aggregated beneath it in corresponding asymmetry (Fig. 2). Asymmetrical regenerates developed
from the acentric blastemata (Thornton, 1960a). Since, however, many
sensory nerve fibers enter the apical cap as it forms these could have
been moved along with the cap to the new acentric location. Blastemal
cells might use nerve fibers as "highways" in migrating to the stump tip
(Singer, 1959; Singer et al., 1964) and if significant numbers of these
pathways, the sensory nerves, were deviated by the movement of the
apical cap, then the asymmetrical aggregation of blastemal cells would
be a result of their movement along these "bent" nerves and not due to
an activity of the apical cap. The discovery by Yntema (1959a), that
limbs which have developed without nerves (aneurogenic limbs) can regenerate typically, provided an experimental design to resolve the problem. When apical caps were moved to asymmetrical positions at the tip
of aneurogenic limb stumps, blastemal cells still aggregated beneath these
caps in corresponding asymmetry (Thornton and Steen, 1962). An apical
cap transplanted to the base of a limb blastema could also induce a secondary outgrowth of blastemal cells which developed into an accessory
limb or limb part (Thornton and Thornton, 1965). When skin from an
innervated larval limb is grafted in place of skin of an aneurogenic limb
stump, regeneration fails. Thus regeneration-competent mesoderm of the
aneurogenic limb requires a stimulus from regeneration-competent skin.
This is clearly demonstrated by grafting mesodermal tissues from an
innervated limb in place of mesodermal tissues of an aneurogenic limb
stump. In this combination, an apical cap is formed and blastemal cells
accumulate beneath it to produce a typical regenerate (Steen and Thornton, 1963). Thus the importance of skin, particularly the apical cap, in
limb regeneration was given added emphasis. The nature of the activity
CHARLES S. THORNTON
cap yet did not penetrate through it to affect mesodermal cells in the
inner core of the limb stump. Apical cap formation and accumulation
of blastemal cells were prevented by ultraviolet treatment (Thornton,
1958). Application of ultraviolet light is an unnatural circumstance and
might conceivably interfere with limb metabolism in unknown (and
undetected) ways. Moving the apical cap to an asymmetrical position
seemed to offer the promise of an unequivocal answer to the cap's possible influence on blastemal cell aggregation. Would an acentric cap
induce an acentric blastema beneath it? To test the possibility, the
wound epithelium of the stump tip was induced to move across and, thus,
heal an adjacent skin wound. The apical cap was carried along with the
migrating epidermis, of which it was a part, so that it came to lie at one
side of the limb tip where blastemal cells aggregated beneath it in corresponding asymmetry (Fig. 2). Asymmetrical regenerates developed
from the acentric blastemata (Thornton, 1960a). Since, however, many
sensory nerve fibers enter the apical cap as it forms these could have
been moved along with the cap to the new acentric location. Blastemal
cells might use nerve fibers as "highways" in migrating to the stump tip
(Singer, 1959; Singer et al., 1964) and if significant numbers of these
pathways, the sensory nerves, were deviated by the movement of the
apical cap, then the asymmetrical aggregation of blastemal cells would
be a result of their movement along these "bent" nerves and not due to
an activity of the apical cap. The discovery by Yntema (1959a), that
limbs which have developed without nerves (aneurogenic limbs) can regenerate typically, provided an experimental design to resolve the problem. When apical caps were moved to asymmetrical positions at the tip
of aneurogenic limb stumps, blastemal cells still aggregated beneath these
caps in corresponding asymmetry (Thornton and Steen, 1962). An apical
cap transplanted to the base of a limb blastema could also induce a secondary outgrowth of blastemal cells which developed into an accessory
limb or limb part (Thornton and Thornton, 1965). When skin from an
innervated larval limb is grafted in place of skin of an aneurogenic limb
stump, regeneration fails. Thus regeneration-competent mesoderm of the
aneurogenic limb requires a stimulus from regeneration-competent skin.
This is clearly demonstrated by grafting mesodermal tissues from an
innervated limb in place of mesodermal tissues of an aneurogenic limb
stump. In this combination, an apical cap is formed and blastemal cells
accumulate beneath it to produce a typical regenerate (Steen and Thornton, 1963). Thus the importance of skin, particularly the apical cap, in
limb regeneration was given added emphasis. The nature of the activity