74
MICHAEL LOCKE
a small wound. The difference is in the point of application of the
stimulus, and the fact that movement of the stimulus causing increase in
diameter can only take place in one direction. This is not due to a simple
concentration gradient, but is an intrinsic property of the tracheae. The
tracheae are polarized with respect to the transport of stimuli causing
growth. The determination of growth and the secretion of the cuticle take
place first at the spiracle and then proceed toward the terminal tracheae.
It would be expected from the polarized conduction of information about
how much to grow that growth should first be completed farthest from the
source of control.
Morphologically and embryologically, the trachea! system is an ingrowth of the surface integument. We might expect that the trachéal
epithelium would have some of the properties of the surface epidermis
which maintains an axial order in the cells. In the polarity of the tracheae
we may have a use for this order, for the transport of information.
6. The Gradient as a Model for Some of the Properties of the Epidermis
The results of the grafting experiments can be described in terms of a
segmentally repeated gradient. This description is no answer in itself.
The gradient is only a model which provides a convenient shorthand
description of the experiments. It is useful in that it enables us to think
about the problems posed by the results with the hope that it may suggest further experiments.
Stumpf (1965a,b) has proposed that the gradient is a gradient of a
diffusible substance. There is no independent evidence for this diffusible
substance, but the model is instructive in that it emphasizes the plasticity
of the epithelium with respect to its position in the gradient. If the transverse ripples represent isoclines, with respect to the quantitative factor
measured by the gradient, then the disturbed regions around and within
transposed grafts are at some intermediate level. We should therefore
think of the arrows in Figs. 26-28 and 30-33 as indicating the direction in
which the isoclines will link up the host and graft patterns as a result of
the juxtaposition of different levels in the gradient. It also helps us to see
why the anterior dominance in isolated grafts (i.e., the ability of the
anterior ripples to take a greater part in producing the pattern in an
isolated graft) is due to the anterior part of a segment being highest in
the gradient.
In other respects the notion of a diffusible molecule is less satisfactory.
The patterns resulting from displaced grafts have considerable stability
from molt to molt. The rearrangement is only at the edges. This rearrangement presumably takes place quickly, immediately after the graft has
been made, although it is only observable after the molt. If the gradient
in the whole graft depended merely upon the initial level of a diffusible
MICHAEL LOCKE
a small wound. The difference is in the point of application of the
stimulus, and the fact that movement of the stimulus causing increase in
diameter can only take place in one direction. This is not due to a simple
concentration gradient, but is an intrinsic property of the tracheae. The
tracheae are polarized with respect to the transport of stimuli causing
growth. The determination of growth and the secretion of the cuticle take
place first at the spiracle and then proceed toward the terminal tracheae.
It would be expected from the polarized conduction of information about
how much to grow that growth should first be completed farthest from the
source of control.
Morphologically and embryologically, the trachea! system is an ingrowth of the surface integument. We might expect that the trachéal
epithelium would have some of the properties of the surface epidermis
which maintains an axial order in the cells. In the polarity of the tracheae
we may have a use for this order, for the transport of information.
6. The Gradient as a Model for Some of the Properties of the Epidermis
The results of the grafting experiments can be described in terms of a
segmentally repeated gradient. This description is no answer in itself.
The gradient is only a model which provides a convenient shorthand
description of the experiments. It is useful in that it enables us to think
about the problems posed by the results with the hope that it may suggest further experiments.
Stumpf (1965a,b) has proposed that the gradient is a gradient of a
diffusible substance. There is no independent evidence for this diffusible
substance, but the model is instructive in that it emphasizes the plasticity
of the epithelium with respect to its position in the gradient. If the transverse ripples represent isoclines, with respect to the quantitative factor
measured by the gradient, then the disturbed regions around and within
transposed grafts are at some intermediate level. We should therefore
think of the arrows in Figs. 26-28 and 30-33 as indicating the direction in
which the isoclines will link up the host and graft patterns as a result of
the juxtaposition of different levels in the gradient. It also helps us to see
why the anterior dominance in isolated grafts (i.e., the ability of the
anterior ripples to take a greater part in producing the pattern in an
isolated graft) is due to the anterior part of a segment being highest in
the gradient.
In other respects the notion of a diffusible molecule is less satisfactory.
The patterns resulting from displaced grafts have considerable stability
from molt to molt. The rearrangement is only at the edges. This rearrangement presumably takes place quickly, immediately after the graft has
been made, although it is only observable after the molt. If the gradient
in the whole graft depended merely upon the initial level of a diffusible