/. Embryol. exp. Morph. Vol. 63, pp. 285-304, 1981
285
Printed in Great Britain © Company of Biologists Limited 1981
Naturally occurring abnormalities
(Bruchdreifachbildungen) in the chelae of three
species of Crustacea (Decapoda) and a possible
explanation
By P. M. J. SHELTON, 1 P. R. TRUBY 1 AND R. G. J. SHELTON 2
From the Department of Zoology, University of Leicester
and the Marine Laboratory, Aberdeen
SUMMARY
Naturally occurring abnormalities (Bruchdreifachbildungen) in decapod crustacean
appendages are described. They are similar to the range of structures experimentally
produced by cutting notches in the sides of insect legs (Bohn, 1965). It is argued that they
result from failure of wounds to heal. Regeneration from a free surface along the proximodistal axis is always in a distal direction. Surfaces regenerating circumferentially can regenerate in either direction around the circumference. Regeneration will proceed until the
two surfaces of the wound meet. Then, where the two surfaces on either side are noncongruent, intervening tissues will be intercalated. This explanation accounts for the range
of structures observed after notching experiments (Bohn, 1965) and seen in crustacean
Bruchdreifachbildungen. The explanation says that regeneration will occur wherever wounds
fail to heal. This avoids the difficulties of the complete circle rule (French, Bryant & Bryant,
1976) and explains why appendages with mirror-image symmetry are often capable of
regeneration.
INTRODUCTION
From time to time, fishermen capture lobsters and crabs with curious,
mirror-image symmetrical lateral outgrowths from their chelae and occasionally
from other appendages. Such abnormalities occur in a variety of animals,
including crustaceans, insects, amphibians, birds and mammals (Przibram,
1921). They have been called' Bruchdreifachbildungen' because they are thought
to result from damage to the original structure and they produce a triplication
distal to the site of injury (Przibram, 1921). In this paper we describe a collection
of abnormal chelae which has been assembled over a period of at least 43
years by the Marine Laboratory, Aberdeen, and provide a possible explanation
for the phenomenon. The deformities resemble the lateral outgrowths some1
Author's address: Department of Zoology, School of Biological Sciences, Adrian
Building, University of Leicester, Leicester LEI 7RH, U.K.
2
Author's address: Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen AB9 8DB,
U.K.
10
EMB 63
Ventral
Dorsal
Internal
Internal
Ventral
Ventral
Propodite
Dactylopodite
Dactylopodite
Propodite/dactylopodite
joint
Propodite/dactylopodite
joint
Propodite
Dactylopodite
N. norvegicus
Homarus gammarus
H. gammarus
Cancer pagurus
C. pagurus
C. pagurus
H. gammarus
3
4
5
6
7
8
9
Internal
Dorsal
Propodite
iV. norvegicus
2
Face of origin
Internal
Segment of origin
Propodite
Nephrops norvegicus
Species
1
No.
Description
Right cutter, fused mirror-image duplication of
the propodite arising from the dactylopodite
joint
Right cutter, fused mirror-image dactylopodites
and joint arising in mid-segment
Right cutter, fused mirror-image propodite
indices arising from the propodite index
Left crusher, fused mirror-image dactylopodite
ends arising near the base of the dactylopodite.
Their teeth appear to be of cutter type
Right cutter, fused pair of dactylopodite ends
arising from the distal part of the dactylopodite
Right chela, fused mirror-image dactylopodites
arising from dactylopodite joint
Left chela, separate mirror-image dactylopodites
arising from the dactylopodite joint
Right chela, fused mirror-image propodite
indices arising from the base of the propodite
extension
Right cutter, a bump arising from the middle of
the dactylopodite
(Specimens 1-11 in this table are illustrated in Fig. 1.)
Table 1
DC
m
r
H
O
2;
00
o
2
c
o
H
X
w
r
to
oo
H. gammarus
H. gammarus
Astacus fluviatilis (Fabr.)
(= A. astacus Linn.)
H. americanus
Palinurus vulgar is
C. pagurus
10
11
12
13
14
15
Internal
Meropodite
Coxopodite
Basipodite
Ventral
Dorsal
Dactylopodite
Carpopodite
Both internal and
external
Dactylopodite
Right cutter, two outgrowths, external one
featureless and terminally damaged; internal
growth divided terminally into mirror-image
dactylopodite ends. Both arise from the
proximal part of the segment
Right cutter, a curly unbranched spike arising
from the middle of the dactylopodite
Described by Bateson (1894, no. 827) and
Przibram (1921, no. 16). A complex example
including a mirror-image appendage with
bifurcation at the distal end of the meropodite
bearing two chelae. An additional bifurcated
appendage of uncertain composition apparently
arising from a second wound on the meropodite
Left cheliped, bearing a mirror-image pair of
limbs with the bifurcation point just below the
carpopodite/propodite joint (Przibram, 1921,
no. 37)
Left penultimate walking leg, two mirror-image
legs joined at the base but separated before the
basipodite/ischiopodite joint arising from the
basipodite (Bateson, 1894, no. 808 and
Przibram, 1921, no. 13c)
Right cheliped, three complete chelipeds derived
from a single coxopodite with the bifurcation
point at the base of the basipodite
(Przibram, 1921, no. 23)
to
oo
I
288 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
times found in cockroach limbs after deep notches have been cut into their
sides during post-embryonic growth (Bonn, 1965). They take various forms
from a small bump to pairs of extra segments. They often exhibit the phenomenon of distal expansion, which has been noted previously in experimentally
formed double-posterior amphibian limbs (Slack, 1977, 1980a) and which is
also seen in some of the lateral regenerates of insect legs (Bohn, 1965). The
fact that crustacean limbs regenerate so readily after autotomy (see Bliss, 1960;
Paul, 1914) and that they can respond locally to produce abnormal lateral
supernumeraries suggests that the crustacean limb might offer a valuable new
system for studying pattern formation. Indeed preliminary reports suggest
that a gradient system, similar to that found in insects, may determine proximodistal organisation of the crayfish leg (Mittenthal, 1978).
We have now examined 11 decapod crustacean limbs showing various
abnormalities. They include specimens from the lobster Homarus gammarus
(L.), the edible crab Cancer pagurus L. and the Norway lobster Nephrops
norvegicus (L.). Together with some of the specimens originally described by
Bateson (1894) and Przibram (1921) there is now a sufficient range of examples
to permit a useful discussion of the cause of the phenomenon. The fact that
Bruchdreifachbildungen occur in such a wide variety of animals suggests that
our findings may be of some general significance.
OBSERVATIONS
All the outgrowths described in this paper are on the terminal or the
penultimate segments of the chela (the dactylopodite and the propodite). However, although this is the most common site for the abnormality, similar outgrowths can occur on antennae (Przibram, 1921, specimen nos 13 a, 136), and
walking legs (Bateson, 1894, specimen no. 808). They can be derived from the
carpopodite (Przibram, 1921, specimen no. 16), the meropodite (Bateson, 1894,
specimen no. 826; Przibram, 1921, specimen no. 37), the basipodite (Bateson,
1894, specimen no. 808) and the coxopodite (Przibram, 1921, specimen no. 23).
There is a natural dimorphism of chelae in decapods (Emmel, 1908) in which
one chela is adapted for 'cutting' and the other for 'crushing'. We found
examples in both cutters and crushers. We adopted the following convention
to identify the origin of the lateral supernumeraries. The chela is flattened
laterally and we call the surface which faces the contralateral chela the internal
face. The outward-facing opposite side is called the external face. The propodite
extends ventrally as the index (or propopodite extension) to make the lower
jaw of the claw structure. The dactylopodite forms the moveable dorsal element
of the claw. Details of the various limbs and their abnormalities including
relevant examples from Bateson (1894) and Przibram (1921) are tabulated
(Table 1) and representative examples are illustrated diagrammatically (Fig. 1).
Where appropriate, we have amplified this description with photographs.
Bruchdreifachbildungen in crustaceans
^ . l i M S * * * ! ! ; . iii'^!i?2.T!Ll(2jauj3y-
Fig. 1. Drawings to show the eleven abnormal claws. For further details see
Table 1. Bar = 5-0 cm.
289
290 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
—
*
I,
*L
*%.
•-.
>
w S k * —,
%
<4' v
*-^^
^
^
•
ri'
^
Fig. 2. External (left) and internal (right) views of specimen no. 2 showing the
origin of the lateral outgrowth. Light coloured featureless cuticle at its base
probably indicates the extent of the original damage. The mid-lateral ridges and
adjacent cuticle dorsal to them are undisturbed. The knobs at the dactylopodite/
propodite joint are typical of the dorsal part of the limb.
Although not all lateral outgrowths have the same structures, there are
several features that most have in common:
(1) All but four of the outgrowths are symmetrical for at least part of their
length (Fig. 1). Often the symmetry takes the form of a pair of mirror-image
structures which may be fused (e.g. nos 1-4) or separate (no. 7) at the base.
Exceptions to this are: no. 9, where the outgrowth is neither symmetrical nor
divided and is in the form of a bump; no. 11, which is a curly spike lacking
clear cuticular markers and thus of uncertain symmetry; no. 10, which has
two lateral outgrowths, one narrow and featureless and one divided at the
end to form a small pair of mirror image dactylopodite ends.
(2) Where features can be identified on the outgrowths they are always
ones that lie distal to the level of the limb from which the outgrowth originates.
This can include joints and more distal limb segments (nos. 2, 12, 15, Table 1,
Fig. 1).
(3) Where an outgrowth is proximally fused, the base of the structure only
Bruchdreifachbildungen in crustaceans
\
291
\ \
Fig. 3. Ventral view of the dactylopodite of specimen no. 5. It shows several
features typical of Bruchdreifachbildungen. The more proximal of the lateral
outgrowths (p) is longer than the other (d). Not all circumferential levels are
represented at the base of the structure. Thus, teeth appear only just below the
bifurcation level. On the distal side of the outgrowth there is a pattern discontinuity
in the form of a bump (arrow), but on the proximal side the outgrowth emerges
smoothly from the dactylopodite. Bar = 10 cm.
Fig. 4. Ventral view of the distal side of the bump of specimen no. 9. It shows a
distortion of the tooth row associated with the polarity reversal on the distal side
of the outgrowth, b, bump; d, dactylopodite. Bar = 50 mm.
Figs. 5, 6. Outgrowth on specimen no. 9 at two moult stages. The same pattern of
teeth (arrows) is recognizable in the cast (Fig. 5) and in the living specimen (Fig. 6).
The general appearance of the outgrowth is virtually unchanged from moult to
moult. Bars = 5-0 mm.
292 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
B
e
el
e2
e3
e4
e5
fl
f2
:
f3
f4
f5
•4
^
f
g
. el fl
e2 f2
e3 f3
. e4 f4"
e5
f5.
h
i J
hi
h2
g3 h3
g4 • h4
g5
h5
el
e2
e3
e4
f3 g3 h3
f4 g4 h4
i3- j3
i4 j4
fl
f2
g3. f3
g4 f4
g5
fl gl
f2 g2
gl
g2
gl
gl
g2
hi
h2
e V f3. g3 h3
e4 f4 g4 h4
g3 f3
g4
g5
f5
il jl
12 j2
i3
i4
" •
j3
j4
•OR.
fl
f2
e5 f5
e4 f4
e3
e4
f4
f5
gl
g2
g3
g4
g5
hi
h2
h3
h4
h5
g5 h5 i5
g4 h4 i4
f3 g3 h3
f4 g4 h4
i3
i4
j5
j4
j3
j4
Fig. 7. Regions of the limb cylinder can be arbitrarily defined in terms of circumferential and proximodistal coordinates (position values) (A) (see French et al.
1976). Different shaped notches (B, C) can expose edges in which either one or
both sets of position values are arranged serially. A transverse cut exposes serial
circumferential values only (D). Such a surface will always regenerate distally
irrespective of stump polarity (D, E). A cut parallel to the proximodistal axis (F)
exposes serial proximodistal values. It is proposed that regeneration of circumferential values may be in either direction around the circumference (G, H).
includes features from the same side of the circumference as that from which
it originates. This is particularly clear in no. 2, where the knobs around the
propodite/dactylopodite joint of the outgrowth are typical of the dorsal part
of the limb (Fig. 2). In other cases, where there are no clear markers at the
site of origin, structures characteristic of other parts of the circumference are
never represented at the base of the outgrowth. For instance, teeth, which are
found on the side of the limb furthest from the outgrowth (e.g. nos. 3 and 4),
are not present in the basal region of the structure. More distal regions of
symmetrical outgrowths include features from further round the circumference
so that at the bifurcation point where the outgrowth divides into two, two
complete circumferences occur (Figs. 1, 3). We refer to this situation as
distal expansion after Slack (1980«).
Bruchdreifachbildungen in crustaceans
c
d
e
f
g
h
i
j
k l
293
m
n
o
p
q
r
s
m
n
o
p
q
r
s
m
n
o
p
q
r
s
B
c d e
Q
Distal regeneration^
c d e f g
c
o
n
d e f g h i ' q p o n m n o p q
r s
Intercalation
E
Fig. 8. A bump may be explained with reference to the proximodistal axis only.
Proximodistal coordinates showing the position of the notch (A). The cut surfaces
at the top of the notch are held apart (B). Distal regeneration occurs from both
free cut surfaces (C). Regenerated tissues meet and fuse (D). When non-congruent
values i and q meet, intercalation of intermediate values occurs to produce a
bump (E). Note that the values m to q are represented three times, the centre set
being a mirror-image of each of the outer sets.
(4) Outgrowths can occur on many points of the chelae, including the carpopodite (no. 12), the propodite (e.g. no. 2) and the dactylopodite (e.g. no. 4).
Jn some instances the outgrowth arises from the joint at the base of the
dactylopodite (nos. 6, 7). From these observations and the previous data
showing that similar supernumeraries have been recorded on walking legs and
virtually all limb segments (we have been unable to find an example of an outgrowth on the ischiopodite) it appears that they can arise from any point on the
proximo-distal axis of a limb including the intersegmental membranes of joints.
(5) Outgrowths can occur on the internal (nos. 1, 5), external (no. 10),
dorsal (nos. 2, 4) and ventral (nos. 3, 8) faces of the limb.
(6) The bifurcation point can occur at different levels along the proximodistal axis of the outgrowth. It may be at the base with little (no. 14) or no
(no. 7) fusion, or it may be in the next segment up the series. Thus in one
case (no. 13) the origin of the structure is the meropodite, the carpopodite is
fused and the two elements are separate only at the level of the carpopodite/
propodite joint. Small outgrowths consist of bumps (no. 9) or spikes (no. 11)
and are undivided.
294 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
Fig. 9. Production of two sets of circumferential values in the side of the limb after
a deep notch. The diagram shows the outline of the notch as seen from above with
intercalation across the middle and free regeneration (arrows) to produce two sets
of circumferential values. This will produce two complete lateral regenerates
without fusion. Shaded area = open wound.
(7) The distal side of the base of the outgrowth often has a pattern discontinuity where it meets the main part of the limb. If there is a clear pattern
at this point, it is symmetrical about this discontinuity. For instance, there is
a ' V pattern in the row of teeth in no. 9 (Fig. 4). More often it is simply
a bump or groove in the cuticle (nos. 4, 5, Figs. 1, 3).
(8) The proximal side of the outgrowth is often longer than the distal side
and this results in the outgrowth pointing distally (no. 3).
(9) Two specimens with lateral outgrowths moulted while in captivity. The
new cuticle of no. 10 is a little damaged so that the tip of the internal lateral
supernumerary is broken and lost. This was the most interesting feature of
the limb because it was formed into a pair of mirror-image dactylopodite tips
(Fig. 1). Nevertheless, comparison of the structure before and after ecdysis
suggests that other aspects of the pattern are the same from moult to moult.
This is confirmed by no. 9 which is still alive and has moulted in captivity.
Here the abnormality is in the form of a bump on the ventral edge of the
dactylopodite (Figs. 1, 5). The shape of the structure is almost the same in
the cast and in the living specimens, and the tooth pattern remains unchanged
Fig. 10. A notch seen from above cutting the proximodistal axis at a shallow
angle. Free regeneration (solid arrows) of values in the reverse direction around
the circumference proceeds to a point where, after fusion, intercalation (dotted
arrows) produces two sets of circumferential values. The resulting structure will
be fused at the base and will bifurcate at a more distal level. Dashed lines in D join
points having the same circumferential values. (E) Plan view of the resulting
structure as it would appear from the side.
Bruchdreifachbildungen in crustaceans
295
296
P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
(Figs. 4, 5). From the fact that decapods can regenerate a complete limb
within a moult cycle (Bliss, 1960) it seems likely that the lateral supernumeraries
are formed within a single moult cycle. Our observations suggest that thereafter the major pattern elements remain stable.
DISCUSSION
As the claws were all recovered from wild specimens the cause of the
deformities remains uncertain. They are unlikely to be due to a genetic abnormality in the pattern forming mechanism, because that would probably result
in much more widespread deformities. Damage during embryogenesis or
damage to a limb bud reforming after a limb had been autotomized would
also tend to produce more widespread effects. Much more likely is damage
to the limb during fighting, possibly while the limb is still soft after moulting,
followed by regeneration. Shortly after moulting the cuticle of H. gammarus
is very brittle. Squeezing a limb with forceps at this stage causes splits in the
exoskeleton and the damaged region can be pulled away easily (Truby, unpublished observations). When two decapods fight they often grasp each other
by the chelae. It is easy to imagine how such behaviour could produce localized
damage to the claws or their intersegmental membranes. Support for this
hypothesis comes from the observation that the abnormalities have been found
found mainly on the chelae and that most cases involve outgrowths on the
claws or preceding segment. It is, of course, possible that such abnormalities
occur with similar frequency on more proximal segments but that major
deformities there increase the likelihood of autotomy.
The idea that damage causes the outgrowths is supported by the views of
other workers. Huxley (1884) reported lateral outgrowths on crayfish limbs
and suggested that they are due to regeneration following limb damage in
newly moulted animals. Bodenstein (1953) concluded that naturally occurring
triple leg structures in insects are caused by a distal portion of the leg being
partially broken off followed by distal regeneration from both the proximal
and distal wound surfaces. Similarly, Przibram (1921) concluded that Bruchdreifachbildungen, similar to the ones we have described, are caused by regeneration from both the proximal and distal sides of a wound in the side of
the limb. There is now direct evidence from experiments with cockroaches to
support the general principle that lateral regenerates are caused by wounding
(Bohn, 1965). Bohn cut V-shaped notches in the ventral and dorsal faces of
tibiae of Leucophaea maderae (Fabr.). Although the wound normally healed
perfectly or left only a small bump, some of the ventral notches gave a range
of Bruchdreifachbildungen of the same type that we have described for
Crustacea. These may include raised bumps at the site of the injury, mirrorimage lateral outgrowths of tibial tissue, similar structures but also including
the tibia/tarsal joint and some tarsal structures terminally. In some cases
Bruchdreifachbildungen in crustaceans
297
regeneration resulted in the formation of all proximodistal levels including the
limb tip. In such structures there is a range of types from those where there is
a bifurcation at the tip to form two separate sets of tarsal claws to those where
the bifurcation point is at the base of the structure before the first joint. The
dorsal notches never produced more than short outgrowths of tibial tissue.
This difference between the effects of ventral and dorsal wounding may not
reflect a physiological difference between cells at the two sites for the following
reasons. First, there are reported cases of naturally occurring dorsal outgrowths
from insect legs showing regeneration as far as the limb tip (Przibram, 1921,
specimen nos. 173a, b). Second, in the crustaceans, which are probably built
according to similar rules, complete outgrowths can occur at any point on
the circumference (see above). It will be shown that our explanation for the
phenomenon depends on failure of wounds to heal. Differences in the curvature
of the limb surface may well affect this process. We think that differences in
regeneration behaviour at different points on the insect leg may be due to
some mechanical factor. It is important to establish this point because, according
to a polar coordinate model (French et al. 1976), one would expect similar
types of cellular behaviour at all points on the circumference. If, however,
cells are specified with reference to a Cartesian system of coordinates, one
might expect cells on opposite sides of the limb to exhibit different types of
behaviour. For instance, they may be able to regenerate only in one direction
along a particular axis (see for instance Slack, 1980a, b).
Another special case of 'wounding' giving rise to pattern duplications or
triplications is known in Drosophila imaginal discs. Here, temperature-sensitive
cell-lethal mutants can be used to produce localized cell death in the discs
(Girton & Russell, 1980). Following such damage to the leg discs, structures
remarkably similar to those described by Bohn (1965) for the cockroach have
been produced in Drosophila (see Girton & Bryant, 1980).
Regarding the general problem of the mechanism generating Bruchdreifachbildungen, there has been no really satisfactory explanation. Why are the
results so variable and why does a notch cause the phenomenon infrequently?
There have been numerous studies on the insect leg and from, them certain
consistent facts emerge. First, when tissues from different proximodistal levels
are recombined, tissues normally separating those levels are regenerated by
intercalation. Cells forming the regenerate are derived from both proximal and
distal faces (Bohn, 1976). A similar pattern of intercalary regeneration follows
when tissues from different points around the circumference are confronted
(French, 1978). Intercalation in that instance is by the shortest possible route
(French, 1978) so that a slightly damaged limb repairs itself rather than
regenerates a mirror-image copy. These findings have led to the formulation
of the 'clockface' or polar coordinate model for explaining distal regeneration
(French et al. 1976). Complete distal regeneration occurs when a complete
circumference is exposed or can be formed by intercalary regeneration from
298 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
sections of the circumference. According to the formal model (French et al.
1976) distal transformation occurs only when these conditions are met. However, distal regeneration can occur in other circumstances. In insects, experimentally created mirror-image symmetrical lateral limb outgrowths are capable
of some regeneration (French, 1976a). In amphibians, mirror-image symmetrical
limbs show varying abilities to regenerate. In the axolotl, double-posterior
limbs give complete distal regeneration when amputated (Slack & Savage,
1978). In the newt Notophthahnus viridescens, amputated double-half limbs
show partial regeneration. Here the distal regeneration is elicited by confrontation of non-congruent circumferential values (Bryant & Baca, 1978). In this
case only a few rounds of intercalation are necessary to resolve all pattern
discontinuities. However, Bryant & Baca (1978) still maintain that a complete
circumference is necessary for total distal regeneration. In insects, after telescoping experiments where congruent proximodistal tibial levels are combined,
lateral regenerates can form if there is partial failure of the host/graft junction
to heal (French, 1976a). Our interpretation of all these phenomena is that
regeneration follows wherever wounds fail to heal. Where a complete circle
forms, healing is effectively prevented by intercalation across the clockface and
the production of an unresolvable point at the limb tip (French, 1976a).
Healing is known to be inhibited when cells from different positions of an
axis are confronted in insect grafting experiments (Niibler-Jung, 1977). This
can lead to rounding up or even rejection of the graft. According to our hypothesis, amputated mirror-image symmetrical limbs regenerate because mechanical factors prevent the exposed parts of the circumference from coming together. To explain Bruchdreifachbildungen it is necessary to assume that
regeneration occurs when wounds fail to heal and that, where complete circles
are formed, distal regeneration will continue until terminal structures have
formed. The clockface model by itself fails to explain the range of structures
formed after notching (Bohn, 1965). It predicts only two outcomes. When the
notch is shallow the limb should repair itself, when it is deep two completely
separate laterals should be produced with no fusion at the base. In the cases
we have described, there is only one instance (no. 7) where the two supernumeraries are separate. In the others, the bifurcation point is a considerable
distance from the base. This was also true of Bohn's (1965) examples in the
cockroach. In addition, examination of some of our specimens shows that the
original damage was localized to much less than half of the circumference.
A clear case is no. 2 (no. 1 is also derived from a small wound). This shows a
symmetrical propodite/dactylopodite joint and a mirror-image pair of dactylopodites arising from the dorsal side of the propodite. The base of the outgrowth occupies no more than 20 % of the circumference of the main limb axis.
In addition, the raised mid-lateral ridges which occur on the internal and
external faces are undisturbed (Fig. 2). The base of the regenerate, which
shows mirror-image symmetry, consists of tissues normally occurring in the
Bruchdreifachbildungen in crustaceans
299
dorsal third of the limb. This argues that notches extending much less than
half-way round the circumference can produce lateral regenerates. According
to the clockface model, damage to a small region of the circumference could
cause laterals to form if positional values are not evenly distributed around
the circumference. Local damage at a site where the values are clustered
could expose more than half the circumferential levels. However, in that case
the supernumeraries should be separate at the base (see below). In all the
cases but one (no. 7), that we have described, the laterals are fused at the
base. Finally, the clockface model does not explain how extra circumferential
positional values are intercalated to cause the common phenomenon of distal
expansion.
Conditions necessary for the production of Bruchdreifachbildungen
Except in the case of the production of two completely separate mirrorimage laterals it is necessary to assume that the wound remains open after
notching. Although the notch may be sealed with a clot of haemolymph, the
edges of the wound may remain apart for a considerable period of time after
injury. Where physical contact of the two sides of confronted tissues fails to
occur, each side may regenerate independently of the other. When proximal
and distal levels of a cockroach limb are telescoped together, both faces usually
contribute to the intercalary regenerate, with the distal face forming more
proximal levels and the proximal face forming more distal levels (Bohn, 1976;
French, 1976a). However, if the two cut surfaces fail to establish cellular
contact, both surfaces behave like distally amputated limbs and, even with
congruent grafts, the proximal face completes the limb and the distal face
regenerates a mirror-image (along the proximodistal axis) of the original graft
(Bohn, 1965; French, 1976a). Thus, although distal regions can give rise to
more proximal ones, when the surface is free, a cut surface always regenerates
more distal parts of the proximodistal axis. There is no evidence concerning the
direction of regeneration around the circumference when a particular circumferential level is exposed and prevented from joining the other side of the wound.
However, we believe that it can regenerate and that it can regenerate in either
direction around the circumference. Only by making these assumptions can
all Bruchdreifachbildungen be explained. According to the shape of the notch
and position of cells on the cut edge, the exposed faces may behave like distally
amputated surfaces or exposed parts of the circumference. If the cells along
the cut surface have serial circumferential values then regeneration will produce
more distal structures. If they have serial proximodistal values then regeneration
will result in new circumferential values being formed (Fig. 7). In a situation
where the surface is at an angle to both axes, they will both have serial values.
Therefore regeneration will proceed along both axes. In the following account,
for simplicity, we have considered only regeneration along one of the two
axes. Thus for a steep-sided notch we have considered regeneration along the
300 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
proximodistal axis and for notches with a shallower angle we have considered
regeneration around the circumference. Later we will describe what happens
when regeneration along both axes is considered at the same time.
The production of bumps or spikes undivided at the tip
Spikes or bumps are likely to arise where the two sides of the notch are
at an acute angle to one another so that each surface behaves like a distally
amputated limb. In this case we will consider regeneration along the proximodistal axis only. The proximal and distal faces of a notch may fail to fuse after
notching because of the mechanical factors involved. For instance the remaining
intact side of the limb will sometimes hold the two edges apart. Following the
rule that a free cut end always regenerates distally (see above), both proximal
and distal sides of the wound will regenerate in a distal direction. Since the
two faces of the notch confront each other the regenerating faces will eventually
meet (Figs 7 & 8). At the base of the notch the two faces will meet almost
immediately. Consequently there will be no great discrepancy of proximodistal values at the fusion point. Further out along the sides of the notch,
regeneration will proceed a considerable way until the two sides meet. This
results in the confrontation of tissues from significantly different proximodistal levels. Intercalation of intervening values will follow to produce a stable
bump (Fig. 8) or spike (specimen no. 11) which would persist unchanged from
moult to moult as observed in specimen no. 9 (Figs. 5, 6). Clearly the nature
and size of the bump would depend on the geometry of the original notch.
Observable features of our specimens are consistent with this explanation.
Distal to the base of the outgrowth there is often a pattern discontinuity in
the form of a groove (no. 4) or a bump (Fig. 3) while on the proximal side
the junction between the main limb and the sidegrowth is not visible. This is
consistent with distal regeneration from the two sides of the notch which
produces a polarity reversal on the distal side but not on the proximal side
(Fig. 4). We have noted that the lateral outgrowths often point distally (specimen no. 11 and many of the mirror-image divided structures) (Fig. 1). This
follows from our explanation because the proximal side of the wound has to
form more distal levels before reaching the tip than the distal side.
A feature of the theory is that it does not necessarily involve consideration
of the circumferential positional values. Assuming that each side of the notch
regenerates independently until fusion, we expect the newly regenerated cells
to derive their circumferential values from the free cut surface just as they
do when a distal amputation occurs. When the two sides of the wound meet
during regeneration the circumferential values should be approximately in
register.
Bruchdreifachbildungen in crustaceans
301
The production of two complete lateral regenerates without fusion
The most complete mirror-image laterals, unjoined at the base and consisting
of all levels distal to the wound site, require a different explanation. It is that
the notch penetrated more than halfway through the appendage. In this case
we do not believe that opposite sides of the wound are necessarily held apart.
Fusion of the two lateral faces, where they are closest together at the base of
the notch, results in the intercalation of intervening circumferential values by
the shortest possible route across the middle of the damaged aiea. This creates
two sets of circumferential values on the side of the limb (Fig. 9). The wound
will effectively remain unhealed at the centre of each set because of the
unresolvable points there. Distal regeneration will follow to completion.
Bruchdreifachbildungen fused at the base and showing distal expansion
Wherever the two lateral supernumeraries are completely separate at the
base, the inference must be that the notch extended more than halfway across
the circumference. If the notch extends less than halfway and the damage is
followed by the two lateral edges of the wound coming together, then the
shortest intercalation rule (French et al 1976) requires the missing circumferential values to be intercalated. The result is local repair without lateral
supernumeraries. This is probably the normal outcome of local damage to a
restricted part of the circumference. Nevertheless in some of our specimens
the original damage was highly localized on one side of the limb and yet
laterals were produced. In these cases the Bruchdreifachbildungen are fused
at the base and distal expansion results in complete separation of the two
laterals at a more distal level. In this case we believe that the original notch
was shallow and/or had sections of the lateral wound surface approximately
parallel to the proximodistal axis of the limb. It is assumed that the wound
remains unhealed and that regeneration begins at the lateral edges. We also
assume that such an exposed edge may regenerate in either direction around
the circumference. Each edge will regenerate without reference to the other
until the two sides finally meet (Fig. 10). After the lateral edges meet, we predict
intercalation by the shortest route around the circumference if there is a
confrontation of non-congruent values. This could have three possible outcomes. First, the two edges can regenerate towards each other. In this case
there will be repair of the wound without lateral regenerates because, when the
two sides meet, we expect near or complete congruences of values. Second, the
two edges can regenerate values in the reverse direction (Fig. 10). If such
regeneration proceeds sufficiently far, values more than halfway round the
circumference from the centre of the original wound can be formed at the
exposed edges. When they meet, intercalation at the interface will occur until
two complete sets of circumferential values have been created. Following
our previous reasoning, the inherent instability of this situation allows distal
302 P. M. J. SHELTON, P. R. TRUBY AND R. G. J. SHELTON
regeneration to completion. Note that, at the base of such a structure, the two
laterals will each consist of less than half of the circumferential values and
they will be joined together in mirror image symmetry. The distal expansion
can be explained by the well established intercalation rule and does not require
special new rules (see for example Slack, 1980#, b). The third possibility is
that on one side of the wound, regeneration proceeds in one direction and
that on the other side, it is in the reverse direction. This would not result in
lateral regenerates because, assuming equal rates of regeneration from each
edge, the relative difference in circumferential values will be approximately
the same as the difference between the original exposed surfaces. However,
intercalation after fusion will result in a bump at the site of the notch.
So far we have considered regeneration along each of the two axes separately
and we have chosen ideal examples where the exposed surface is nearly parallel
to one of the axes and cuts the other at an acute angle. However, in most
notches the wound will be more or less U shaped when viewed from the side.
So, in some places along the wound margin a given axis will be cut at a shallow
angle and in others it will be cut at an acute one. For this reason the resulting
outgrowth will have features of bumps and of laterals with distal expansion.
Thus, an outgrowth may have a polarity reversal (with respect to the proximodistal axis) at the distal side of the lateral's base, but also show distal expansion
(Fig. 3).
CONCLUSION
In this discussion we have attempted both to explain the phenomenon of
Bruchdreifachbildungen and to provide an explanation for distal regeneration
which does not depend upon the complete circle rule (French et al. 1976)
alone. This is because the complete circle rule demonstrably fails in the case
of the distal regeneration of mirror-image symmetrical limbs (French, 1976;
Slack & Savage, 1978) and because by itself the complete circle rule cannot
explain the range of structures found in the Bruchdreifachbildungen we have
described for crustaceans and Bohn (1965) has observed in insects. After
due consideration, we concluded that the main cause of regeneration in limbs
is the failure of wounds to heal. Distal regeneration following the formation
of complete circles is just a special case of wounds failing to heal. It may be
brought about because of the unresolvable point at the centre of a complete
circumference or because cells on opposite sides of the circumference have
sufficiently different surface properties that healing is inhibited. Our hypothesis
could explain why amputated mirror-image symmetrical limbs are capable of
distal regeneration. All that is required is regeneration of all values distal to
the cut before the cells at the circumference come together at the tip. This is
quite possible; most fields are small at the time of determination (Wolpert,
1969; Crick, 1970) and subsequent cell divisions merely increase the size of
the organ. In addition, just the geometry of an amputated cylinder is not
Bruchdreifachbildungen in crustaceans
303
conducive to healing at the tip. An important point deriving from our argument
is that it is immaterial which circumferential values are represented in the
wounded surface. For this reason we can explain the regeneration of anomalous
mirror-image symmetrical lateral supernumeraries in amphibians after 180°
rotations of the limb (Maden, 1980). In that case we propose that, after the
operation, the wound fails to heal over part of the circumference due to some
obstruction (possibly a blood clot) or perhaps to a misalignment of host and
graft tissues. The partial set of values intercalated around the wound will have
mirror-image symmetry and distal regeneration will result in these values being
perpetuated until either the most terminal levels are formed or the wound
finally heals. In conclusion, we can summarize our explanation as follows.
A cut free surface will sequentially regenerate values along the axis orthogonal
to the cut. Any exposed part of the circumference may begin to regenerate so
long as the surface remains free. In the case of the proximodistal axis regeneration
from a free surface will always proceed in a distal direction while in the
circumferential axis it can proceed in either direction around the circumference.
Clearly our ideas are speculative and they are based on a rationalisation of
phenomena exhibited by a set of naturally occurring monstrosities. Nevertheless,
they provide a possible way out of the complete circle impasse and they suggest
the need for further experiments and accurate observation of cell behaviour
at wound surfaces.
We thank Angela Chorley for drawing the chelae and preparing Fig. 1. We are also
grateful to Dr P. J. Hogarth for helping with our literature search. P.R.T. was supported by
an M.R.C. studentship.
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(Received 15 October 1980, revised 27 January 1981)