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
55
no part in the final structure, and (3) spontaneous assembly, the creation
of an ordered structure as a result of the intrinsic properties of the
components in a given environment. In discussing the method of growth
we should separate two problems: (1) initial formation and (2) the
continued production or reproduction of the membrane. Once the first
patches of cutieulin are present, the growth could proceed by accretion
at the edges or intussusception between the components already in position. Given that the ordered structure has arisen, it might then act as a
template for partially ordered components which may themselves have
arisen by spontaneous assembly. All three methods of growth may thus
take place depending upon the phase of growth being considered.
c. The Initial Formation of the Cutieulin. The first possibility is ruled
out for the initial formation of the cutieulin, since, unlike plasma membranes, the cutieulin arises de novo, not from preexisting cutieulin or any
other membrane. The second two alternatives are more difficult to
separate. If the plasma membrane were acting as a template, we might
expect to see cutieulin membrane precursors settling on it, until condensed enough to peel away. This is not the appearance of the growing
edges. The cutieulin arises near to the plasma membrane, but not near
enough for it to act as a template. It is difficult to determine exactly
how near the cutieulin and plasma membrane may be, since the region
between them is partly filled by the dense material presumed to be
precursors passing through on their way to form the cutieulin. The
cutieulin seems to merge insensibly with its precursors. The third hypothesis is thus most probable for the initial formation of cutieulin. Precursors
pass through and concentrate above particular regions of the plasma
membrane where they spontaneously aggregate into plaques of cutieulin.
This hypothesis is also favored by the finding that the cutieulin may
very early show the transverse striae of the porous pattern. These rodlike structures could even be the main components out of which the
membrane is built. One could visualize a two-stage process in which
there is first an assembly of elongated particles which then align side
by side with one another.
d. The Growth of the Cutieulin. Once patches of cutieulin are present,
the question arises whether growth occurs by accretion at the edges or
by intussusception all over. There are several points suggesting that
growth is at the edges. Perhaps most importantly, the wisps of dense
material connect the plasma membrane only with the edges of patches
of cutieulin, not with the entire surface. The way in which the cutieulin
spreads to occupy the future surface area for the next stage larva also
FIG. 20. The cutieulin in a fully formed tracheole. Micropores about 30 Â across
can be seen in the cutieulin. X 320,000.
FIG. 21. The cutieulin at the surface of fully formed epicuticle. X 130,000.
55
no part in the final structure, and (3) spontaneous assembly, the creation
of an ordered structure as a result of the intrinsic properties of the
components in a given environment. In discussing the method of growth
we should separate two problems: (1) initial formation and (2) the
continued production or reproduction of the membrane. Once the first
patches of cutieulin are present, the growth could proceed by accretion
at the edges or intussusception between the components already in position. Given that the ordered structure has arisen, it might then act as a
template for partially ordered components which may themselves have
arisen by spontaneous assembly. All three methods of growth may thus
take place depending upon the phase of growth being considered.
c. The Initial Formation of the Cutieulin. The first possibility is ruled
out for the initial formation of the cutieulin, since, unlike plasma membranes, the cutieulin arises de novo, not from preexisting cutieulin or any
other membrane. The second two alternatives are more difficult to
separate. If the plasma membrane were acting as a template, we might
expect to see cutieulin membrane precursors settling on it, until condensed enough to peel away. This is not the appearance of the growing
edges. The cutieulin arises near to the plasma membrane, but not near
enough for it to act as a template. It is difficult to determine exactly
how near the cutieulin and plasma membrane may be, since the region
between them is partly filled by the dense material presumed to be
precursors passing through on their way to form the cutieulin. The
cutieulin seems to merge insensibly with its precursors. The third hypothesis is thus most probable for the initial formation of cutieulin. Precursors
pass through and concentrate above particular regions of the plasma
membrane where they spontaneously aggregate into plaques of cutieulin.
This hypothesis is also favored by the finding that the cutieulin may
very early show the transverse striae of the porous pattern. These rodlike structures could even be the main components out of which the
membrane is built. One could visualize a two-stage process in which
there is first an assembly of elongated particles which then align side
by side with one another.
d. The Growth of the Cutieulin. Once patches of cutieulin are present,
the question arises whether growth occurs by accretion at the edges or
by intussusception all over. There are several points suggesting that
growth is at the edges. Perhaps most importantly, the wisps of dense
material connect the plasma membrane only with the edges of patches
of cutieulin, not with the entire surface. The way in which the cutieulin
spreads to occupy the future surface area for the next stage larva also
FIG. 20. The cutieulin in a fully formed tracheole. Micropores about 30 Â across
can be seen in the cutieulin. X 320,000.
FIG. 21. The cutieulin at the surface of fully formed epicuticle. X 130,000.