76
MICHAEL LOCKE
of the sternites in third-instar larvae and implanted in similar positions
in the tergites. Some were rotated through 180° to induce the discontinuity pattern; others were implanted with normal orientation as controls.
The changes in area could be followed by examining the exuviae. The
differences between the isolated cuticle and the normally oriented transplant were most marked in the adult. In the rotated squares, the transplant was reduced to a few bristles with no pigmented cuticle. In those
with normal orientation, the transplant was marked by an oval of pigment with normally spaced and oriented bristles. These results are in
agreement with the hypothesis derived from a study of growth in tracheae,
that epithelial continuity is necessary for the quantitative control of
growth. The epithelial continuity must be of a particular kind, being part
of an axial gradient in the normal tergite.
Isolated pieces of cuticle may have satisfied their capacity for uniting
with cells of the same axial level by forming a concentric pattern, but
they fail to grow harmoniously with the rest of the animal. Satisfaction
of the capacity for completing the pattern is alone insufficient to induce
normal growth ; the integument must also be in the gradient of the whole
segment. The quantitative control of growth is therefore not coincident
with the behavior of the cells described as a gradient. If the amount of
growth depended upon a bloodborne factor alone, the converse might
have been expected. That this is not so implies that in the normal animal
the cells receive information about the growth to be made in some other
way, presumably through the epithelium as in tracheae.
The stimulus to grow in diameter originates from the nodes in the
lateral tracheae. The nodes are homologous with the intersegmental membranes; they are junctions between different segments. It is therefore of
interest that grafts containing the intersegmental membrane tend to survive whatever their orientation in the animal, unlike similar grafts without the intersegmental membrane. This may mean that the intersegmental membranes, like the nodes, are in some sense organizers for
growth (Locke, 1959a, 1960b). Thus the intersegmental membranes, and
through them individual segments, may have some independence in
growth. This idea is supported by the finding of Krishnakumaran and
Schneiderman (1964) that some segments molted once and some twice
in isolated adult abdomens of Cecropia made to molt by being parabiosed
to Polyphemus pupae.
Stumpf (1965c) has drawn attention to the possible role of particular
levels of the gradient as inducers for specific differentiation. In Galleria
pupae, there are transverse ribs of cuticle in the posterior segments which
occur at particular distances from the intersegmental membranes. She
found that operations on the larva which produced levels appropriate to
the rib position in new locations also induced the formation of new ribs.
MICHAEL LOCKE
of the sternites in third-instar larvae and implanted in similar positions
in the tergites. Some were rotated through 180° to induce the discontinuity pattern; others were implanted with normal orientation as controls.
The changes in area could be followed by examining the exuviae. The
differences between the isolated cuticle and the normally oriented transplant were most marked in the adult. In the rotated squares, the transplant was reduced to a few bristles with no pigmented cuticle. In those
with normal orientation, the transplant was marked by an oval of pigment with normally spaced and oriented bristles. These results are in
agreement with the hypothesis derived from a study of growth in tracheae,
that epithelial continuity is necessary for the quantitative control of
growth. The epithelial continuity must be of a particular kind, being part
of an axial gradient in the normal tergite.
Isolated pieces of cuticle may have satisfied their capacity for uniting
with cells of the same axial level by forming a concentric pattern, but
they fail to grow harmoniously with the rest of the animal. Satisfaction
of the capacity for completing the pattern is alone insufficient to induce
normal growth ; the integument must also be in the gradient of the whole
segment. The quantitative control of growth is therefore not coincident
with the behavior of the cells described as a gradient. If the amount of
growth depended upon a bloodborne factor alone, the converse might
have been expected. That this is not so implies that in the normal animal
the cells receive information about the growth to be made in some other
way, presumably through the epithelium as in tracheae.
The stimulus to grow in diameter originates from the nodes in the
lateral tracheae. The nodes are homologous with the intersegmental membranes; they are junctions between different segments. It is therefore of
interest that grafts containing the intersegmental membrane tend to survive whatever their orientation in the animal, unlike similar grafts without the intersegmental membrane. This may mean that the intersegmental membranes, like the nodes, are in some sense organizers for
growth (Locke, 1959a, 1960b). Thus the intersegmental membranes, and
through them individual segments, may have some independence in
growth. This idea is supported by the finding of Krishnakumaran and
Schneiderman (1964) that some segments molted once and some twice
in isolated adult abdomens of Cecropia made to molt by being parabiosed
to Polyphemus pupae.
Stumpf (1965c) has drawn attention to the possible role of particular
levels of the gradient as inducers for specific differentiation. In Galleria
pupae, there are transverse ribs of cuticle in the posterior segments which
occur at particular distances from the intersegmental membranes. She
found that operations on the larva which produced levels appropriate to
the rib position in new locations also induced the formation of new ribs.