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
47
they enlarged and the main growing points became fewer and restricted
to the edges in the valleys.
The simple hypothesis that the plasma membrane is the prime mover
and takes on the surface pattern, later secreting the cuticulin above
it, is clearly disproved by the origin of the cuticulin on microvilli. However, many surface patterns are larger than microvilli, and it could be
that the cell still directs these patterns by determining the shape of the
surface which the microvilli cover. If this were so, we should expect that
the first step in the formation of the pattern would be an alteration in the
shape of the cell to become a mold for the cuticulin. This has not been
seen. In all the patterns examined, if the cell surface altered in pattern,
it did so after the formation of the cuticulin and not before.
The observations are consistent with the view that the pattern is
determined primarily by the growing cuticulin. This is not to say that
the cells have no influence. They may exert their effect through the
initial shape of the cell, the rate at which the cuticulin grows, and the
final increase in surface area produced. For example, the difference in
pattern between the cuticle over the wax glands (Fig. 12) and that over
the rest of the surface (Fig. 13) may be that result of different rates of
synthesis over the surface of each cell. Both patterns have a cellular basis
(Figs. 14 and 15). Each proturberance, or polygonal crater-studded
depression, arises from a single cell. In the epidermis of the wax gland,
most cuticulin deposition is at the edges of the cell, so that the junctions
between cells are elevated in raised rims around concave centers (Fig.
14). Further expansion in the central area then has to take place against
the resistance of the cell and might only be relieved by buckling at a
finer level to produce the crater-shaped tubercles. We can distinguish
two orders or levels of pattern formation. First, the polygonal areas arise
with depressed centers. Later, the finer, second order pattern of cratershaped tubercles appears. In the other surface cuticle, greatest synthesis
of cuticulin is in the center of each cell, so that this bulges out, becoming increasingly convex to form the tubercle (Fig. 15).
The cause of the four patterns studied seems to be the expansion and
subsequent deformation of the cuticulin. The differences in pattern could
be attributed to: (1) the shape of the surface when the cuticulin is
secreted, (2) the distribution of the cuticulin over this surface, i.e., the
rate at which it is deposited over particular parts of a cell, and (3) the
stresses and subsequent buckling which result from 1 and 2.
b. Patterns Formed by Interactions between the Cuticulin and Intracellular Fibers. Epidermal derivatives, such as scales, hairs and bristles
may prove suitable experimental material for investigating the general
problem of the mechanism by which the form of cells may be determined.
Picken (1949), studied the formation of scales in Ephestia by X-ray
47
they enlarged and the main growing points became fewer and restricted
to the edges in the valleys.
The simple hypothesis that the plasma membrane is the prime mover
and takes on the surface pattern, later secreting the cuticulin above
it, is clearly disproved by the origin of the cuticulin on microvilli. However, many surface patterns are larger than microvilli, and it could be
that the cell still directs these patterns by determining the shape of the
surface which the microvilli cover. If this were so, we should expect that
the first step in the formation of the pattern would be an alteration in the
shape of the cell to become a mold for the cuticulin. This has not been
seen. In all the patterns examined, if the cell surface altered in pattern,
it did so after the formation of the cuticulin and not before.
The observations are consistent with the view that the pattern is
determined primarily by the growing cuticulin. This is not to say that
the cells have no influence. They may exert their effect through the
initial shape of the cell, the rate at which the cuticulin grows, and the
final increase in surface area produced. For example, the difference in
pattern between the cuticle over the wax glands (Fig. 12) and that over
the rest of the surface (Fig. 13) may be that result of different rates of
synthesis over the surface of each cell. Both patterns have a cellular basis
(Figs. 14 and 15). Each proturberance, or polygonal crater-studded
depression, arises from a single cell. In the epidermis of the wax gland,
most cuticulin deposition is at the edges of the cell, so that the junctions
between cells are elevated in raised rims around concave centers (Fig.
14). Further expansion in the central area then has to take place against
the resistance of the cell and might only be relieved by buckling at a
finer level to produce the crater-shaped tubercles. We can distinguish
two orders or levels of pattern formation. First, the polygonal areas arise
with depressed centers. Later, the finer, second order pattern of cratershaped tubercles appears. In the other surface cuticle, greatest synthesis
of cuticulin is in the center of each cell, so that this bulges out, becoming increasingly convex to form the tubercle (Fig. 15).
The cause of the four patterns studied seems to be the expansion and
subsequent deformation of the cuticulin. The differences in pattern could
be attributed to: (1) the shape of the surface when the cuticulin is
secreted, (2) the distribution of the cuticulin over this surface, i.e., the
rate at which it is deposited over particular parts of a cell, and (3) the
stresses and subsequent buckling which result from 1 and 2.
b. Patterns Formed by Interactions between the Cuticulin and Intracellular Fibers. Epidermal derivatives, such as scales, hairs and bristles
may prove suitable experimental material for investigating the general
problem of the mechanism by which the form of cells may be determined.
Picken (1949), studied the formation of scales in Ephestia by X-ray