P A T T E R N S I N T H E I N T E G U M E N T OF I N S E C T S
75
substance, we should expect that the rearrangement would rather quickly
affect the whole graft and not be restricted to the edges in this way. The
diffusion model is also unsatisfactory in explaining the related phenomenon of polarity in tracheae with respect to the distribution of the
factor influencing the furture diameter. The tracheae are polarized with
respect to the movement of this factor, and the movement cannot be reversed by surgical procedures which would alter a diffusion gradient (Fig.
39). Although the notion of diffusion within the gradient may be helpful
in enabling us to visualize the problem, the results of the experiments
can be predicted from the properties of a gradient of any sort. They cannot be adduced as evidence for the existence of one kind of gradient made
up of a concentration difference of a particular diffusible molecule.
The falling sand model of Lawrence (1965) is more instructive. Lawrence studied the pattern of bristles and hairs in Oncopeltus and also
found evidence for a repeating gradient. He visualized his results in a
gradient of sand grains. When "grafts" in a slope of sand grains are
transposed or disoriented, the grains fall to a new stable position with
the formation of new contours or isoclines. His concept differs from the
diffusion model above in that the new patterns are stable; the force of
gravity causing the grains to fall is balanced by the friction between the
grains. The model exactly describes his experiments and those discussed
here. There is an interaction at the edge of a graft predictable from the
gradient, but outside this region both graft and host have a stable pattern. This model draws attention to the local stability of the gradient.
A cell may have its position in the gradient redetermined as at the margin
of a graft, but thereafter it again becomes stable. For a possible morphological correlate to account for this stability see Section II,B,9 below.
7. The Significance of the Gradient Organization
It is assumed that the ripple pattern of the cuticle is caused by and
reflects some similarly oriented mechanism within the cells of the epithelium. The question arises whether any other property of these cells obeys
similar rules.
The trachéal epithelium is polarized with respect to the control of
growth. If this phenomenon is related to the repeating axial gradient of
the tergites, then transposed or rotated grafts of integument with patterns
isolated from those of the host ought to behave like tracheae without
tissue connections, molting normally but failing to increase in size.
The growth of rotated grafts with isolated patterns has been followed
making use of the darkly pigmented sternal cuticle as a marker (Locke,
1959a, 1960b). The sternal cuticle differs from the tergal in retaining
bristles in the adult, but it has similar transverse ripple marks, and
grafts to the tergites take satisfactorily. Squares were cut from the center
75
substance, we should expect that the rearrangement would rather quickly
affect the whole graft and not be restricted to the edges in this way. The
diffusion model is also unsatisfactory in explaining the related phenomenon of polarity in tracheae with respect to the distribution of the
factor influencing the furture diameter. The tracheae are polarized with
respect to the movement of this factor, and the movement cannot be reversed by surgical procedures which would alter a diffusion gradient (Fig.
39). Although the notion of diffusion within the gradient may be helpful
in enabling us to visualize the problem, the results of the experiments
can be predicted from the properties of a gradient of any sort. They cannot be adduced as evidence for the existence of one kind of gradient made
up of a concentration difference of a particular diffusible molecule.
The falling sand model of Lawrence (1965) is more instructive. Lawrence studied the pattern of bristles and hairs in Oncopeltus and also
found evidence for a repeating gradient. He visualized his results in a
gradient of sand grains. When "grafts" in a slope of sand grains are
transposed or disoriented, the grains fall to a new stable position with
the formation of new contours or isoclines. His concept differs from the
diffusion model above in that the new patterns are stable; the force of
gravity causing the grains to fall is balanced by the friction between the
grains. The model exactly describes his experiments and those discussed
here. There is an interaction at the edge of a graft predictable from the
gradient, but outside this region both graft and host have a stable pattern. This model draws attention to the local stability of the gradient.
A cell may have its position in the gradient redetermined as at the margin
of a graft, but thereafter it again becomes stable. For a possible morphological correlate to account for this stability see Section II,B,9 below.
7. The Significance of the Gradient Organization
It is assumed that the ripple pattern of the cuticle is caused by and
reflects some similarly oriented mechanism within the cells of the epithelium. The question arises whether any other property of these cells obeys
similar rules.
The trachéal epithelium is polarized with respect to the control of
growth. If this phenomenon is related to the repeating axial gradient of
the tergites, then transposed or rotated grafts of integument with patterns
isolated from those of the host ought to behave like tracheae without
tissue connections, molting normally but failing to increase in size.
The growth of rotated grafts with isolated patterns has been followed
making use of the darkly pigmented sternal cuticle as a marker (Locke,
1959a, 1960b). The sternal cuticle differs from the tergal in retaining
bristles in the adult, but it has similar transverse ripple marks, and
grafts to the tergites take satisfactorily. Squares were cut from the center