PATTERNS IN THE INTEGUMENT OF INSECTS
69
within itself, the center of the graft, farthest from the host, being highest
in the gradient. The appearance of some of these grafts is similar to the
disturbed pattern figured by Piepho (1955). Marcus (1962) also describes
grafts in which the surroundings show a concentric succession of structures normally found in the axis of a segment.
4- Determination and Rede termination of the Gradient
The question arises whether the epidermal cells retain permanently
their gradient properties and their level within the gradient. When burns
are made involving cells with the potential for forming several color and
cuticle patterns, there is a centripetal displacement of the patterns into
the wounded region. The gap is repaired by cells migrating in from all
sides, carrying their patterns with them (Wigglesworth, 1937, 1940). The
cells maintain their potential to form these color and cuticle patterns in
a new situation. A different result is obtained with respect to their position in the gradient if burns or excisions are made in the fifth stadium
within the area destined to form the adult ripple pattern (Locke, 1959a).
Within the ripple pattern there is complete regeneration. If we think of a
burn as making a hole in the gradient, then the hole can be filled in without trace. These experiments on the ripple pattern did not show that the
cells repairing the wound came from different levels in the gradient, as we
should expect from Wigglesworth's experiments. They only showed that
the pattern, and by inference the gradient, could regenerate. Some other
experiments suggest that migrating cells may change their level in the
gradient.
If cells are killed at any time up to 9 days after feeding (i.e., 9 days
after the initiation of molting), the ripple pattern regenerates with normal orientation and an undisturbed pattern (Fig. 36). If the burn is made
exactly as the new epicuticle is being formed, the surrounding cuticle is
normal, but the burned region is repaired with a thin white patternless
cuticle or a melanised blood clot. After this time, the new cuticle has
already been laid down and the ripple pattern is undisturbed, although
the cells below may have been destroyed. We can show that the cells in
these burns migrate in from all sides just as in Wigglesworth's experiments. There is a very short time before the epicuticle is laid down when
burns result in a centripetal pattern (Fig. 24). The ripples point towards
the center of the burn as though the cells below are carrying with them
an orientation appropriate to their original position. We may suppose
that at this critical time these migrating cells show their orientation in
the gradient, but are still capable of moving. If the burn had been
slightly earlier, they would have had time to move and assume the
orientation appropriate to their new level. Thus, cells which take part in
69
within itself, the center of the graft, farthest from the host, being highest
in the gradient. The appearance of some of these grafts is similar to the
disturbed pattern figured by Piepho (1955). Marcus (1962) also describes
grafts in which the surroundings show a concentric succession of structures normally found in the axis of a segment.
4- Determination and Rede termination of the Gradient
The question arises whether the epidermal cells retain permanently
their gradient properties and their level within the gradient. When burns
are made involving cells with the potential for forming several color and
cuticle patterns, there is a centripetal displacement of the patterns into
the wounded region. The gap is repaired by cells migrating in from all
sides, carrying their patterns with them (Wigglesworth, 1937, 1940). The
cells maintain their potential to form these color and cuticle patterns in
a new situation. A different result is obtained with respect to their position in the gradient if burns or excisions are made in the fifth stadium
within the area destined to form the adult ripple pattern (Locke, 1959a).
Within the ripple pattern there is complete regeneration. If we think of a
burn as making a hole in the gradient, then the hole can be filled in without trace. These experiments on the ripple pattern did not show that the
cells repairing the wound came from different levels in the gradient, as we
should expect from Wigglesworth's experiments. They only showed that
the pattern, and by inference the gradient, could regenerate. Some other
experiments suggest that migrating cells may change their level in the
gradient.
If cells are killed at any time up to 9 days after feeding (i.e., 9 days
after the initiation of molting), the ripple pattern regenerates with normal orientation and an undisturbed pattern (Fig. 36). If the burn is made
exactly as the new epicuticle is being formed, the surrounding cuticle is
normal, but the burned region is repaired with a thin white patternless
cuticle or a melanised blood clot. After this time, the new cuticle has
already been laid down and the ripple pattern is undisturbed, although
the cells below may have been destroyed. We can show that the cells in
these burns migrate in from all sides just as in Wigglesworth's experiments. There is a very short time before the epicuticle is laid down when
burns result in a centripetal pattern (Fig. 24). The ripples point towards
the center of the burn as though the cells below are carrying with them
an orientation appropriate to their original position. We may suppose
that at this critical time these migrating cells show their orientation in
the gradient, but are still capable of moving. If the burn had been
slightly earlier, they would have had time to move and assume the
orientation appropriate to their new level. Thus, cells which take part in