/. Embryol. exp. Morph. Vol. 51, pp. 1-26, 1979
Printed in Great Britain © Company of Biologists Limited 1979
The specification of metameric order in the insect
Callosobruchus maculatus Fabr. (Coleoptera)
I. Incomplete segment patterns can result from constrictioninduced cytological damage to the egg
By JITSE M. VAN DER MEER 1
From the Department of Zoology, Catholic University, Toernooiveld, Nijmegen,
The Netherlands
SUMMARY
Eggs of the pea-beetle Callosobruchus were divided into two at different stages of development. Both fragments were allowed to develop into partial larvae. The segment patterns of
normal and partial larvae are described using cuticular markers of cell differentiation. To
study the contribution of cytological damage to the segment gap phenomenon three different
types of constriction were performed: complete and incomplete permanent constriction and
complete temporary constriction.
Changes in the structure of the egg can produce absence of segments resulting from two
different effects. First, partial absence of segments results from a decreased egg circumference
in the constriction region and involves the disturbance of a morphogenetic process (dorsal
closure). Secondly, cytological damage can result in a gap between two arrays of segments.
The loss of segments in the gap occurred in two different ways. In a spatial segment gap the
two arrays of segments were physically discontinuous, whereas in a non-spatial gap the segments bordering the gap were juxtaposed in a physically continuous cuticle.
The extent to which the gap phenomenon can be attributed to cytological damage is
discussed. We also discuss, on the basis of certain dorsal defects, a possible stepwise specification of the dorsal transverse cuticular pattern.
INTRODUCTION
Pattern formation is the development of spatially integrated sets of differentiated cells. Insects are very suitable to study pattern formation because of their
simple metameric organization. Moreover, each larval or adult segment can be
distinguished from all others by cuticular markers (bristles, hairs, stigmata, etc.).
Epigenetic metamerization in insects can be described either in terms of two
morphogenetic signals (Krause & Sander, 1962; Kalthoff, 1976; Sander, 1976)
or in terms of only one such signal (Meinhardt, 1977). Sets of blastoderm cells
would become instructed to form particular segments by the position-dependent
absolute value of one signal or by the ratio between two such signals.
1
Author's address: Division of Membrane Biology and Biochemistry, Institute of Experimental Pathology, German Cancer Research Centre. D-6900 Heidelberg, Federal Republic of
Germany.
2
J. M. VAN DER MEER
When an insect egg is divided into two fragments in an early stage, the resulting
anterior and posterior partial larvae each lack a number of segments near the
constriction region. As fragmentation is made at successively later stages the
number of missing segments becomes smaller. This gap phenomenon is interpreted to result either from the interference by the constriction with the transport
of morphogenetic signal(s) (Sander, 1976), or from signal redistribution after
constriction and its interference with cellular commitment (Meinhardt, 1977).
The result would be the loss on either side of the constriction of a number of
specific instructions and their corresponding segments.
However, the loss of segments may also be caused by cytological damage to
the egg or to a putative mosaic of prelocalized segment determinants. Various
arguments make this possibility unlikely (Herth & Sander, 1973). Moreover,
development is often normal if the constriction is immediately released and the
ooplasm of the two egg halves re-fuses (Alleaume, 1971; Sander, 1975, 1976;
Yogel, 1977), or if re-fusion is forced by puncturing the barrier produced by
constriction (Schubiger, Moseley & Wood, 1977). Since neither of these procedures is likely to reduce the disturbance of the egg by the constriction, it was
concluded that the segment gap is not due to egg cell damage.
In this paper, however, we describe that in Callosobruchus different types of
constriction can damage the egg resulting in modified segment patterns.
MATERIALS AND METHODS
Adults. Pea-beetles {Callosobruchus maculatus Fabr., syn. Bruchus quadrimaculatus Fabr.) were provided by the Pest Infestation Control Laboratories.1
A stock culture was kept on green peas {Pisum sativum) in the dark at 30 °C
and 70 % relative humidity (r.h.) (Brauer, 1925; Brauer & Taylor, 1936; Howe
& Currie, 1964). Development from egg deposition through four larval instars
till the emergence of the adult then lasts about 4-5 weeks (Howe & Currie,
1964: table IV). Culture tubes and oviposition boxes were treated with 5 % of
methyloxybenzoate (nipagine) in 96 % ethanol to prevent growth of fungus,
which serves as food for the dust-louse {Liposcelis divinatorus Miill.) and for
mites (Tyrophagusputrescentiae Schrank.). [A warning: prolonged exposure to
beetles and/or pea powder may result in severe allergic reactions.] For description
of adults (Fig. 1 a, b) and their egg laying behaviour see Brauer (1925), South gate,
Howe & Brett (1957) and Howe & Currie (1964).
Eggs. The egg's shape is that of one longitudinal half of a hen's egg (Fig. 1 d).
The average egg dimensions are: 700 /on long, 200/mi wide and 150 jam largest
height. Large numbers of eggs were obtained by collecting about a hundred
young females in a box covered with 0-5 mm mesh plankton gauze. Males should
be added to stimulate oviposition. Oviposition was initiated by adding brown
beans {Phaseolus vulgaris) on which the beetles were placed with a brush.
Staging accuracy was ±15min. The first egg collection could be used for
1
London Road, Slough SL3 7HJ, Buckinghamshire, U.K.
Pattern formation in insect embryogenesis
Fig. .1. Dorsal views of a female (a) and male (b) of Callosobmchus: length about
3-4 mm. The female abdomen extends posteriorly from under the elytrae and shows
a white median stripe while the elytrae are characterized by four dark spots (a).
The males have a short abdomen and a less pronounced coloration (b). (c) Constricted egg seen through the window of the constriction apparatus. The anterior
and posterior isolate each contain a partial embryo, (d) Cellular blastoderm
stage: vertical bar shows the constriction levels, (<?) Egg constricted at 54 % e.l. (arrow)
during stages NM2/NM16-32. The plane separating the posterior blastoderm from
the anterior yolk mass shows a slight anterior shift to 58 % e.l. (/) Egg constricted
at 87 % e.l. during stages NM4-8/SBy. Ooplasmic displacement from the smaller
into the larger isolate produces an almost empty anterior isolate. All eggs, except (c)
are shown from the ventral side with their anterior poles upward. The egg in (c) is
shown from the dorsal side. The frayed outline of the egg is the result of an adhesive
substance, secreted by an accessory gland of the female oviduct. It is poured out
over the egg during deposition. The dark outline of the chorion (d) and the mark left
in the chorion by the razor blade (arrows in e and/) are the reference points for
measuring constriction levels.
4
J. M. VAN DER MEER
experiments, because when the beetles were deprived of a laying substrate, the
eggs did not start embryonic development while still in the oviduct.
After 30 min the beans were put in tap water of 30 °C to facilitate egg removal
and to prevent the chorion from hardening. Eggs were removed under a dissecting microscope with a piece of blunted razor blade in a holder and collected in
embryo dishes containing insect Ringer (7-5 g NaCl, 0-35 g KC1, 0-21 g CaCl2
made up to 1000 ml distilled water and sterilized). They were either used
immediately or stored until the desired stage of development was reached.
Alternatively, beans with eggs were stored at the desired temperature in a
desiccator containing a glycerol/water mixture giving 70 % r.h. (64:100 (v/v)
glycerol in water, Johnson, 1940).
Damaged eggs including those with a small rift in the vitelline and egg cell
membrane only (internal extraovate) were discarded.
Constriction procedure. Eggs were oriented with their flat dorsal side on slides
covered with plastic foil, using a hair-loop and a dissecting microscope; they
were stored in a moist chamber (100 % r.h.) until used. Eggs were fragmented
transversely with a pinching apparatus modified after Sander (1971) (Fig. 1 c).
First, the position of the razor blade was adjusted to the desired level of constriction with an ocular micrometer. Next, the razor blade was lowered without
making abrupt movements to prevent the bursting of eggs and the sudden
streaming of ooplasm from the smaller into the larger egg fragment (Fig. 7).
Complete as well as incomplete, permanent constrictions and complete,
temporary constrictions were carried out. The absence of individual yolk globules
moving over the glittering edge of the razor blade was a criterion for complete
constriction.
Incomplete constriction was achieved by orienting pieces of copper wire, with
mean diameters of 58 /mi (54-62) and 116 /im. (111-121) at several places in the
row of eggs. The razor blade was lowered until the pieces of copper wire were
slightly pressed, giving slit sizes of about 30 or 65 % of the maximal egg height
(150 /.cm). These are maximum estimates, because the underlying plastic foil may
slightly swell in water-saturated air or in fluorocarbon oil.
With permanent constriction the eggs had to be covered with a thin film of
fluorocarbon oil (Voltalef 10S) to prevent dehydration and excessive rusting of
the razor blade. Structural changes in the chorion leading to increased permeability may result from the high stretching forces applied by constriction. In
Protophormia eggs such changes produce translucency of the chorion (Herth &
Sander, 1973). Next, the apparatus with the eggs was stored at 30 °C and 100 %
r.h. After 7-8 days a cuticle is formed, as shown by the heavily pigmented head
parts. After deconstriction, constriction levels were determined individually
for each egg by expressing the distance measured from the outer edge of the
chorion at the posterior pole (0 % e.l.) to the mark left by the razor blade in the
chorion as a percentage of the total egg length (e.l., micrometer divisions:
Fig. \d).
Pattern formation in insect embryogenesis
5
The following is essential to temporary, complete constriction. The beans,
covered with eggs, were immersed in water immediately upon oviposition to
retain chorionic flexibility. The constricted eggs were incubated at 30 °C and
100 % r.h. without fluorocarbon oil. During deconstriction the eggs were
observed under a dissecting microscope and care was taken to avoid abrupt
ooplasmic back-flow (Fig. Ib-e). After collecting the eggs in insect Ringers, they
were left at 20-22 °C for 30 min, to allow re-fusion of anterior and posterior
ooplasm within a relatively short developmental period and subsequently they
were incubated at 30 °C and 100 % r.h.
Normal stages at which constrictions were carried out are: MI (anaphase of
first meiotic division), NMn (nuclear migration stage with n indicating the
number of nuclei in the egg), SBy, SBm and SBo: syncytial blastoderm young
(no cell membranes, protruding pole cells), middle (radial cell membranes being
formed, protruding pole cells) and old (radial cell membranes cut deeply into
periplasm, pole cells less prominent), respectively, CB (cellular blastoderm,
basal cell membranes formed) and YP (ventral plate, first histological differentiation between ventral embryonic and dorsal extra-embryonic blastoderm,
onset of gastrulation, cephalic furrow not yet formed). With temporary constriction, constriction and deconstriction stages will be indicated in that order,
separated by a slant line.
Light microscopy. Larval segment patterns were analysed with phase contrast
optics. Partial larvae were pricked with sharp tungsten needles and subsequently
transferred to small tubes containing an acetic acid: glycerol mixture (4:1, v/v)
in which the internal organs largely dissolve after an incubation at 60 °C overnight. Next, the larvae were mounted under a small coverslip in Hoyer's mixture
and incubated overnight at 45 °C to complete solubilization (Van der Meer,
1977). Finally, the coverslips were ringed with euparal.
Cuticular markers used for identification of segments. The normal first instar
larva consists of 19 segments: 5 cephalic, 3 thoracic and 11 abdominal segments
(Fig. 2). Each lateral half of the larva was divided into 6 longitudinal rows of
markers, connected by broken lines in Fig. 2.
The head. From front to back the successive cephalic markers are: labrum,
antennae, mandibulae, maxillae and labium, all of which, except the last, could
be easily recognized (Figs 2,4a). The labial segment contains a small lip situated
in between the maxillae but somewhat posterior to them. The labial lip is
occupied by two small bristles with a number of chaeta-like structures in between.
VMR-bristles are situated in a left and right row between the lip and the
anteroventral segment border of the first thoracic segment (Fig. 2b2). Each row
consists of three bristles, a large, a small and a medium-sized one from front to
back. It is not clear whether the bristles belong to the maxillary or the labial
segment. Dorsally, the head is characterized by the head capsule (Brauer, 1946).
Externally the encapsulated part of the head is overlapped by a chitinous plate:
the cephalic shield. This becomes heavily sclerotized some hours before hatching.
Mx
Fig. 2(«) and (6). For legend see opposite.
Mxp
St
LRl
Csh.
DLRr
100 Mm
100 Aim
T
A6
VMRr
A7
(c 2 )
100/im
Fig. 2. Dorsal (a), ventral (b) and lateral (c) views of normal first-instar larvae of Callosobruchus, showing cuticular markers. Thin broken
lines indicate the following bristle rows: DMR, dorsomedian row; DLR, dorsolateral row; LR, lateral row; St, stigmatal row; VLR,
ventrolateral row; VMR, ventromedian row. Index T or 'r' indicates left or right. Abbrev.: Ax through An, abdominal segments; Ant,
antenna; Csh, cephalic shield; He, head capsule; Lb, labium; Lbr, labrum; Md, mandibula; Mx, first maxilla; Mxp, palpus maxillaris;
Sp, spine of Ax; St, stigma; Tx, T2 and T3, first, second and third thoracic segment, respectively. Broken lines with longer dashes (a2, b2)
indicate cuticular ruptures (t) caused by the flattening of the larva during preparation. Phase-contrast. Details: see text.
l)
Mx. Mxp.
He
DLR1
8
J. M. VAN DER MEER
It projects laterally and posteriorly over the top of the head and possesses
marginate teeth on its anterior part.
The thorax. Each segment is characterized ventrally by a pair of small legs with
a row of microchaetae in between. The first segment is characterized by a small
row of microchaetae posterior to the main row and by the distance between the
short and long VMR-bristles. Dorsolaterally it displays a specific pattern of
bristles and knobs. A stigma is situated bilaterally on the border between Tx and
T2. T3 is characterized by one or two small LR-bristles.
The abdomen. Segment Ax is characterized dorsally by the distance between
the short DMR- and the long DLR-bristle; ventrolaterally by the spine and the
long LR-bristle. A2 through A4 and A 5 through A7 do not have characteristic
differences to distinguish the segments within either group. However, A4 is
distinguished from A 5 by the additional long LR-bristle. The small LR-bristle
is lacking in A9. This segment is further characterized ventrally by the short
distance between the two small bristles of VMR and VLR, and dorsally by the
short distance between the one DMR-bristle and the two DLR-bristles. Stigmata
are also absent. A 8 can be distinguished from A9 by the longer distance between
the two small bristles of VMR and VLR, and from A7 by the number of VMRbristles (one and three, respectively). A10 is distinguished by the absence of
ventral microchaetae and long DLR-bristles, while A u only has the slit-shaped
opening of the hindgut. Thus almost all the segments of a partial larva could be
identified individually, except the two groups of abdominal segments.
RESULTS AND CONCLUSIONS
Three types of constriction have been carried out. First, eggs were constricted
completely and permanently, that is until after the development of partial larvae
in the two egg fragments was completed. Secondly, with incomplete, permanent
constriction, the razor blade was screwed down such that during the constriction
the two egg fragments remained connected by a small slit of ooplasm. Finally,
eggs were constricted completely but temporarily for different time intervals
starting sometime during nuclear migration and with deconstriction not later
than at the cellular blastoderm stage.
1. Permanent, complete constriction
Permanent, complete constrictions were carried out to see whether the gap
phenomenon (see Introduction) also occurs in Callosobruchus eggs. Eggs were
constricted between 5 and 100 % e.l. at the following stages (number of eggs in
parentheses): first meiotic division (MI: 600), two nuclei (NM2: 754), 16-32
nuclei (NM16-32: 998), syncytial blastoderm young (SBy: 595) and middle
(SBm: 430), cellular blastoderm (CB: 414) and ventral plate (VP: 545). The
total proportions of analysable partial larvae from anterior, posterior and both
fragments were: 17, 31, 44, 37, 28,48 and 48 %, respectively. The rest of the eggs
Fig. 3. (a) Lateral view of anterior partial larva (complete, permanent constriction at 49 % e.l., stage CB) ending in segment Ax. The dorsal
cuticular markers of T2 through Ax are absent. The ventral rows of microchaetae have extended in dorsal direction and are medially split
into a left and right row. In T3 a local multiplication of a long bristle is seen on one side of the partial larva, (b) Dorsal view of posterior
partial larva (constriction at 52% e.l., stage NM16-32) ending in segment A4. Details: see text. Abbrev.: see Fig. 2. Phase-contrast.
(a2)
ia
10
J. M. VAN DER MEER
either did not develop or produced partial larvae which were not analysable
(max. 4 %).
A segment gap of fairly constant size was found after constriction till the
young syncytial blastoderm (SBy). The gap rapidly became smaller during
subsequent syncytial and cellular blastoderm stages, so that virtually no segments
were missing at the ventral plate stage. A quantitative analysis of these results
will appear in a subsequent paper (Van der Meer & Miyamoto, 1979). Here we
describe only the so-called dorsal defects which are frequently observed in the
segments bordering the constriction, since these have relevance in deciding the
role of damage in producing the gap phenomenon.
Description of partial larvae with dorsal defects
Figure 3 b shows a dorsal view of a posterior partial larva with border
segment A4 and missing dorsal pattern elements, whereas the ventral pattern
elements in question are always present ('ventralized segments'). Dorsal defects
were observed in 41 % of partial larvae and only in segments near the constriction.
They were most frequent where the egg reaches its largest height at about
50-80 % e.l. The dorsal defects can vary in degree, depending on the distance to
the site of constriction. Minimally, only the most dorsomedian structures are lacking, as shown by the DMR- and DLR-bristle groups of A7 which approach
each other and those of A 6 which have disappeared. Pattern elements situated
more dorsolaterally and laterally drop out successively with decreasing distance
from the constriction region. This is shown by the convergence and eventual
fusion of the left and right rows of stigmata (St) and bristles of the lateral rows
(LR) in Fig. 3 b (compare Fig. 2a2). In a few extreme cases all segments of a
posterior partial larva consisted only of ventral rows of microchaetae, on the
ventral as well as on the dorsal side. The circumference of these segments was
decreased, while the scale of the ventral pattern of microchaetae was harmoniously increased, sometimes showing duplication of single bristles (Fig. 3a:
T3).
A lateral view of an anterior partial larva is shown in Fig. 3 a. It terminates
with the first abdominal segment and shows the dorsal extension of the ventral
patterns of segments Tx through Al5 common in segments with dorsal defects.
Fig. 4 c shows the complete fusion of the ventral T3 legs on the dorsal side except
for their tips. In very small anterior partials, fusion of maxillae (Fig. 4b),
mandibulae or antennae is often observed.
Eversion, i.e. the pointing inward of external cuticular structures (Ando,
1955) was also regularly encountered in posterior partial larvae.
Pattern formation in insect embryogenesis
11
Fig. 4. (a) Ventral view of cephalic markers. Bar: 50/tm. (b) Symmetrical fusion
of left and right maxillae (Mxi) in a small anterior partial larva. Dashed line of
bilateral symmetry runs over the fused maxillary palpi. Bar: 10/tin. (c) Dorsal
defects with fused T3 legs (L) and Ax spines (the two accompanying bristles (b) at the
base of Sp, remained separate). Bar: 10/tm. Abbrev.: Fig. 2. Phase-contrast.
2. Permanent, incomplete constriction
It is generally assumed (Sander, 1976) that communication between the two
egg halves is necessary for the formation of a complete segment pattern. Incomplete constriction is not, therefore, expected to result in a segment gap,
because ooplasmic continuity is only partially interrupted. Only damage by the
12
J. M. VAN DER MEER
incomplete constriction to the egg could lead to a segment deficit. To test this,
234 eggs were constricted between 38 and 94 % e.l. at stages MI, NM2 and
NM16-32, using slits of about 50 and 100/tm, and 129 analysable larvae were
obtained. Such eggs often (31 %) developed into partial larvae, physically
separated into an anterior and a posterior fragment. This suggests that the
ooplasmic continuity had been interrupted even though the constriction left a
slit of 50 or 100 jam. In some cases the number of segments from the anterior
and the posterior partial larva together constituted a complete set of segments.
In others, however, one through nine segments were absent.
Incompletely constricted eggs also developed into normal (non-fragmented)
larvae containing the complete set of segments (with proportions of 36 and 30 %
for ooplasmic slits of 100 and 50 /an, respectively).
A third class of seven larvae was obtained in which the segment gap was not
associated with a physical discontinuity in the cuticular pattern. Instead, segments were absent in a physically continuous cuticle, the segments bordering the
gap being juxtaposed.
The latter two classes of results were also found with incomplete, permanent
constriction of Drosophila eggs (Vogel, 1977). The results suggest, that cytological damage can produce the absence of segments when the egg is incompletely constricted.
3. Temporary, complete constriction
If communication between the two egg halves is necessary for the development
of a complete set of segments, a further test of whether the partial or complete
absence of segments can result from cytological damage would be to temporarily
interrupt this communication. This has been done by complete but temporary
constriction of eggs for various time intervals. We observed, that re-fusion of
anterior and posterior ooplasm can occur and that normal larvae developed
mainly after brief constriction. We consider as the best test to the damage
hypothesis re-fused eggs, which were constricted for short time intervals. First,
because as a result of the additional deconstriction the eggs underwent much
more disturbance than permanently constricted eggs (Fig. Ib-e). The assumption here is, that, if the gap is produced by damage only, the additional deconstriction should produce a larger gap than with permanent constriction.
Secondly, re-fusion is also a necessary condition for the restoration of the
putative communication between the two egg halves. Thirdly, because very
brief constriction is unlikely to produce large deviations in the spatial distribution of segmental instructions.
Types of segment gap
Although constriction was not permanent, the egg often developed into a
physically separate anterior and posterior larval fragment. If the number of
segments in two such fragments from the same egg (bipartite development) was
13
Pattern formation in insect embryogenesis
100 jim
St
(a)
1
2
3
~
9
10
11
12
13
14
15
16
17
18 19>|
(b)
Fig. 5. (a) Larva with segments Mx, Lb, Tl5 T2 and T3 (serial numbers 4 through 8)
missing and segments Ant. and Ax physically continuous. Below the larva is a formal
description of the segment pattern, the first segment being the labral. The second
cephalic segment (Ant.) is normally situated between labrum and mandibula, but has
been anteriorly displaced as a result of overturning the head during mounting. Constriction was in stage SBy at 55 % e.l. with a 50 /*m slit, (b) Close-up showing absence
of gross cuticular irregularities in the gap region. Abbrev.: Fig. 2; Fg, fore-gut. Bar:
50 fim.
2
EMB 51
14
J. M. VAN DER MEER
Segment gap
3| |l2
Type
Code
19| Spatial gap
S.'S
S/S
Non-spatial gap S~S
Fig. 6. Types of larvae observed after remporary, complete and permanent, incomplete constriction in Callosobruchus. Horizontal bars are formal descriptions
of segment patterns, with segments indicated by their serial numbers. S, Array of
normally adjoining segments, (a) Normal (n) larva with complete set of segments,
(b) anterior and posterior partial larvae with a gap of segments 4 through 11,
associated with a physical discontinuity in the cuticle, (c) the same as in (b) but
without segments missing, (d) partial larva in which the absence of segments 5
through 9 is associated with a physically continuous cuticle. Details: see text.
less than nineteen, a spatial segment gap is said to be present (Fig. 6 b). This is the
type of gap usually encountered in the literature up to now (Sander, 1976) and
will be designated S/S, the diagonal bar indicating a gap in the segment pattern
associated with a physical discontinuity in the differentiated cuticle. This
symbol is also used if the number of segments in two such physically disconnected
larval fragments is nineteen, i.e. when there is no segment gap.
Not all constricted eggs produced larvae with a clear cuticular discontinuity
in their segment pattern. In this case a number of segments are missing (for
example Mx through T 3 : Fig. 5 a) resulting in the juxtaposition of segments (e.g.
Ant and Ar: Fig. 5a) which in the normal pattern are separated. To distinguish
them from gaps associated with a physical discontinuity in the cuticle, such gaps
will be called non-spatial segment gaps, the designation S ~ S (Fig. 6 d) indicating
a segment gap associated with a physically continuous cuticular pattern.
In addition, multiple gaps infrequently occurred in different combinations
within one larva (Table 1: e.g. S/S/S, S ~ S ~ S or S/S ~ S) or in partial larvae,
where only one of the two fragments developed (Table 1: e.g. anterior partial
S / S / - , posterior partial - / S ~ S).
Finally, segments were sometimes missing in positions far from the constriction
region ('remote constriction effects'). Posterior or anterior segments can be
missing in posterior and anterior partial larvae, respectively. When bipartite
development occurs after equatorial temporary constriction, the most anterior
as well as the most posterior segments can be absent in the partial larvae (Table 1:
e.g. -/S~S/-,-/S/S/-).
15
Pattern formation in insect embryogenesis
Table 1. Types of segment pattern observed after temporary, complete
constriction
Partial larvae
Anterior
Type of
segment pattern
s/-/s
s/s
s~s
-/s/s~s/-/s~ s/-/s/s/-/s/s
-/s~s
-/s~s~s
s/s/s/s/s
s~s~s
s/s~s
s~s/s
Anterior+
posterior
Posterior
A
A
t
n
%
n
227
—
—
—
4
8
—
—
—
(0)
14
690
—
—
87
6
2
—
—
—
—
—
—
—
—
—
(0)
(0)
—
—
—
—
—
—
—
—
—
3
4
1
1
11
1
—
—
—
—
—
0/
/o
1*
23
—
—
3
(0)*
(0)
(0)
(0)
(0)
(0)
—
—
—
—
—
n
4
—
171
243
6
4
4
1
—
2
—
4
1
3
5
1
O
/O
\
(0)
—
6
8
(0)
(0)
(0)
(0)
—
(0)
—
(0)
(0)
(0)
(0)
(0)
Percentages calculated as a fraction of the total number of analysable larvae (2996); those
< 0-5% are given as (0). Not included are complete larvae (see Table 2). A special subclass of
posterior partial larvae (double abdomens) will be dealt with in a forthcoming paper.
* Anterior partial larvae can develop from posterior egg fragments when the constriction is
placed anterior to about 80% e.l. (Fig. If). The missing posterior segments presumably
result from the disturbance of the abdominal extension ('remote constriction effect': see text).
Abbrev.: see Fig. 6 legend.
Cyto logical observations
To correlate the spatial, non-spatial and multiple segment gaps with cytological observations, 863 eggs from temporary constrictions during stages NM2/
NM16-32 between 50 and 80 % e.l. were observed at critical stages of development after deconstriction. These observations were documented for individual
eggs with drawings and photographs. The series showed that usually blastoderm
formation was not retarded as a consequence of constriction.
Early anterior or posterior constriction results in blastoderm formation in the
posterior (Fig. Ig) or anterior fragment alone (unipartite development), because
nuclei are excluded from the complementary fragment (position of the zygote
nucleus: 60± 10 % e.l.). Late equatorial constriction frequently gives rise to a
bipartite blastoderm (Fig. If) resulting in anterior and posterior partial larvae in
the same egg (bipartite development). Eggs with a bipartite blastoderm (Figs.
If 8«) gave rise only to anterior and/or posterior partial larvae. Eggs in which
16
J. M. VAN DER MEER
68% E.L.
Dorsal /"Chonon
p
. Perivitelline space
I Yolk endoplasm
Vitelline membrane +
Cellular blastoderm
(/)
Double CB
Fig. 7. Ooplasmic displacement during constriction and deconstriction and
subsequent events till cellular blastoderm formation in Callosobruchus. Heavy arrows:
movement of razor blade. The size of the arrows in the hatched yolk endoplasm
indicates the extent of its displacement. Thin arrows: successions in developmental events. Due to differences in volume: surface ratio, ooplasm will be forced
from the smaller into the larger egg fragment during constriction (b), (c). This is
more pronounced in the anterior than in the posterior region, because the maximum
egg diameter is reached anterior to the middle. After complete separation (d) the
double layer of vitelline membrane and chorion is not fragmented. The oolemma
becomes continuous around each of the two egg parts (unpublished E.M. observations). With deconstriction ooplasm flows back (<?), resulting in a small shift of the
actual separation plane with respect to the chorionic mark made by the razor blade.
This plane is slightly oblique, because ooplasmic back-flow is somewhat more
extensive on the dorsal side, (a) Egg immediately after oviposition, (b) through
(d): progression of constriction, (e) through (/): deconstricted eggs, (e): egg deconstricted prior to blastoderm formation, (/) same egg with blastoderm formed in
both fragments, (g) egg with posterior unipartite blastoderm and anterior degrading yolk mass (cross hatched), (h) same egg showing ooplasmic clefts prior to
blastoderm formation, (/) anterior and posterior ooplasm, completely re-fused,
(j) incomplete re-fusion leaving an ooplasmic cleftfilledwith cytoplasm. In some cases
it becomes populated with nuclei (k) and then it may remain syncytial or become
cellular (/). Abbrev.: see Fig. 6. Drawings based on in vivo observation of 863 eggs.
anterior and posterior ooplasm re-fused within 30 min after deconstriction
(27 cases) (Fig. 7 e,j, i) developed either into normal larvae or into larvae with
non-spatial gaps. The same was also observed when re-fusion occurred later,
shortly before blastoderm formation (127 cases). In one special class of 28 eggs
the two egg fragments remained divided, after deconstriction, by a transverse
zone situated in the former constriction region (Figs. 1 e,j,k, %b) and containing
Pattern formation in insect embryogenesis
17
Fig. 8. Types of constriction region in temporarily constricted eggs (stages NM48/NM64-128, 50% e.l.) and photographed at the cellular blastoderm stage, {a)
Cellular blastoderm on both sides of the constriction region. Arrows: contact
between the two blastodermal sheets, (b) and (c) Transverse zone (tz) in the constriction region is located perpendicularly to the blastodermal sheet (CB) and is
filled with yolk-free cytoplasm without (b) or with (c) nuclei. The transverse zone in
(c) can be either syncytial or cellular (preliminary E.M. observation). In (b) and (c)
the cellular blastoderm is continuous over the transverse zone. Abbrev.: A, anterior
egg fragment; CB, cellular blastoderm; n, nucleus; P, posterior egg fragment; tz,
transverse zone; y, yolk globules. Bars: 10/tm. Differential interference contrast.
Explanation: see text.
translucent, yolk-free cytoplasm. In all these 28 embryos a blastoderm was
formed around the egg and continuous over the region containing the transverse
zone (Fig. 86,c), because nuclei were present on both sides of the constriction.
These embryos developed exclusively into larvae with non-spatial gaps.
The transverse zone develops from an incomplete re-fusion of anterior and
posterior ooplasm. After deconstriction (Fig. le) a small cleft is left in the
ooplasm, filled with translucent, yolk-free cytoplasm (Fig. 7/). Initially, this
zone may be stabilized by remnants of the oolemma (unpublished E.M.
18
J. M. VAN DER MEER
Table 2. Fractions of normal larvae (n) and larvae with non-spatial
segment gaps (S ~ S) from re-fused eggs obtained with temporary, complete
constriction
Re-fused survivors
Stage of:
A
A
1
\
Constriction
NM2
Tx
T2
An
n (%)
11
10
9
44
75
202
129
58
91
(0)
11
4
5
(2)
0
NM4-8
NM16-32
NM64-128
0
1
2
4
0
1
2
100
0
20
86
9
SBy
3
7
21
0
NM16-32
0
2
8
9
0
4
6
7
0
0
0
9
7
3
0
7
3
1
0
3
1
0
56
228
17
21
80
3
(6)
0
48
6
(1)
0
62
58
21
19
(10)
0
46
18
0
30
57
(3)
Deconstriction
NM2
NM4-8
NM16-32
SBy
NM4-8
NM16-32
SBy
SBm
SBo
CB
SBy
SBo
CB
SBy
SBm
SBo
CB
SBm
SBo
CB
7
10
9
8
69
51
74
99
50
19
19
S~S(%)
0)
(2)
5
0)
28
16
16
Abbreviations: T\, duration of constriction (h 30 °C); T2, time interval between deconstriction and stage CB (h 30 °C). Percentages of analysable survivors from re-fused eggs were
calculated as a fraction of the total number of analysable larvae (An) obtained between 30 and
69% e.l. Re-fusion of anterior and posterior egg halves was restricted to this constriction
interval. Percentages in parentheses: n < 3 .
observations) together with degradative changes in the yolk bordering the
constriction and a difference in viscosity between the cytoplasm and the yolk
globules. Sometimes this zone became populated with nuclei (Fig. 8 c) and either
remained syncytial (Fig. Ik) or became cellular (Fig. 11). Alternatively, it may
remain devoid of nuclei (Fig. Sb).
After some early constrictions it was observed that nuclei could not pass the
former constriction region in eggs, classified as re-fused. Electron microscope
observations (Van der Meer, unpublished) showed that the transverse zone can
be so narrow as to be invisible in the dissecting microscope. This explains,,
upon re-examination of the data, why larvae with non-spatial gaps also occurred
outside the above special class of 28 eggs with visible transverse zones. Therefore, re-fusion was checked at the cellular blastoderm stage, when the various
classes of eggs are clearly recognizable. Non-re-fused eggs then show up by a
Pattern formation in insect embryogenesis
19
double layer of cells (Figs. If, 8 a) at the former constriction region. Actually refused eggs show no interruption in the endoplasm or in the blastoderm (Fig. li).
Any originally invisible transverse zone in incompletely re-fused eggs becomes
visible either by the accumulation of cytoplasm in the zone (bipartite development) or by the absence of a blastoderm in one of the two egg halves (unipartite
development). Wrongly classified eggs were re-classified. These observations
showed that complete re-fusion was mainly confined to constrictions roughly
between 30 and 69 % e.l.
The frequencies of re-fused survivors, separated into normal larvae and larvae
with non-spatial gaps were calculated as fractions of total analysable larvae
within 30-69 % e.l. (Table 2). Re-fused survivors are most frequent (80-90 %)
with brief constriction ( T ^ O h , i.e. 0-5 min) at any stage, except NM48/NM4-8. Their proportion also depends on the stage of constriction (compare
Tx = 4 h between SBy/SBm and NM2/SBy) and decreases with increasing duration of the constriction (Tj), except for NM4-8/NM4-8.
The percentage of normal larvae from re-fused survivors is maximally 91 %
and generally more than 50 % with brief constriction at any stage until after
cellularization of the blastoderm has progressed considerably at the old syncytial
blastoderm stage (SBo; except during NM4-8/NM4-8). It is still 21 % with
brief constriction as late as CB/CB and decreases with increasing Tx.
The percentage of larvae with non-spatial gaps from re-fused survivors starts
very low with very early constriction (NM2/ —) and increases with constriction
starting later during nuclear migration (NM4-8/ —, NM16-32/ —), but not with
both very long and very short time intervals between deconstriction and stage
CB(T2). It reaches a maximum with constriction shortly before stage CB (SBy/ - ) ,
but only with the longer T2 (SBy/SBy, SBm/SBm). However, it is still 16 %
with brief constriction as late as SBo/SBo and CB/CB.
DISCUSSION
We want to discuss the following questions. Do our cytological observations
indicate that the absence of segments after permanent, incomplete and temporary,
complete constriction results from cytological damage ? How is the segment
pattern affected by cytological damage ? How do the spatial and non-spatial
gaps arise ? What are the conditions that prevent cytological damage to occur,
leading to the development of normal larvae ? To what extent can data from
permanent, incomplete and temporary, complete constriction explain the gap
phenomenon as revealed by permanent complete constriction ?
20
J. M. VAN DER MEER
1. Cytological damage produced by temporary constriction can result in the absence
of segments
The origin of non-spatial segment gaps
We have observed that eggs with a transverse zone exclusively developed into
larvae with non-spatial gaps. In vivo observation showed in 5 out of 102 eggs
relatively large clefts in the ooplasm bordering the constriction region (Fig. Ih).
Four of them died, but one developed into a posterior partial larva. After anterior
constriction, for example, such clefts may arise from 'stretching' of the ooplasm
in the constriction region of the posterior fragment, because the ooplasm tends
to flow back in the anterior fragment (Fig. lb-e,h). When the constriction is
situated more equatorially, the ooplasmic rearrangements are less extensive and
much smaller clefts might arise which, in vivo, often remain invisible and do not
impair egg survival. This assumption is consistent with the observation that
non-spatial gaps were found between 30 and 69 % e.l.
Observations in vivo and with the electron microscope (Van der Meer, unpublished) showed the ooplasmic clefts to be filled with translucent, yolk-free
cytoplasm (Fig. 8Z>). Some clefts later could be populated with nuclei (Fig. 8c)
to become either a syncytium or a cellular structure. The question as to how the
segment pattern is affected by the transverse zone is difficult to answer. Sander
(1976) separated segment pattern formation into the distribution of segmental
instructions through the egg (specification), followed by the commitment of
cells to segmental pathways of differentiation. The transverse zone might interfere
either with the specification or the commitment or with both. In the first case
the zone would block signal transport. It is currently an open question whether
signal transport takes place in the periplasm or the yolk endoplasm or both
(Sander, 1976). If normal signal transport would be entirely restricted to the
yolk endoplasm, its separation by a transverse zone might result in depletion and
accumulation of signals on either side. A number of instructions and the
corresponding segments would disappear. The segment gap would be non-spatial
because the continuity of the blastoderm over the transverse zone guarantees the
formation of a continuous cuticle in the segment gap region. However, if signal
transport would be restricted to the periplasm, oolemma or blastoderm, the
transverse zone might draw off a number of instructions from the periphery. If,
subsequently, the transverse zone would not participate in segment formation,
the segmental instructions in it would be lost resulting in a non-spatial gap. If
the transverse zone would also interfere with segmental commitment, the addition
of two complicated effects does not allow any reasonable explanation of the gap
phenomenon.
One origin of non-spatial gaps can be excluded. The two juxtaposed segments
in non-spatial gaps did not meet and fuse secondarily after the formation of two
separate blastoderms in one egg (bipartite blastoderm). First, eggs with a
bipartite blastoderm never produced non-spatial segment gaps. Secondly, the
Pattern formation in insect embryogenesis
21
cuticular irregularities expected with such a growing together were absent
(Fig. 5 b).
The observation of anterior and posterior partial larvae after permanent,
incomplete constriction suggests that a complete ooplasmic separation can result.
This separation may also start with the formation of ooplasmic clefts, as in
temporary, complete constriction. However, here it may proceed with the
physical separation of the anterior and posterior ooplasm. The latter was
observed in Euscelis with narrow 'bottle-neck' ligations (Sander, personal
communication). Although clefts could not be directly observed in Callosobruchus
eggs permanently contained in the constriction apparatus, we believe that the
seven larvae with non-spatial gaps also arose from transverse zones as in
temporary, complete constriction. This may have been the case in Calliphora
(Nitschmann, 1959) and Drosophila (Vogel, 1977) as well.
The frequency of non-spatial segment gaps
The frequency of larvae with non-spatial gaps will depend on factors determining the frequency of transverse zone formation. Among such factors are the
level, stage and duration of constriction as well as factors influencing the
structural stability of the zone. The latter can be remnants of the oolemma
(unpublished E.M. observations) together with degradative changes in the yolk
bordering the constriction, differences in viscosity between the cytoplasm in the
zone and the surrounding yolk globules and the immigration of nuclei into the
zone followed, in some cases, by cellularization.
The near-absence of non-spatial gaps after early temporary constrictions
(Table 2: NM2/ - ) results from the predominance of normal larvae from refused eggs and of anterior and posterior partial larvae. If a transverse zone would
be left, one cannot expect larvae with non-spatial gaps, because early constricted
eggs mostly have nuclei on one side of the constriction only. The near-absence of
non-spatial gaps after constrictions of longer duration and deconstriction shortly
before the cellular blastoderm stage may result from the progressive cellularization on either side of the constriction, which hampers the formation of a transverse zone.
After brief (0-5 min) constriction during blastoderm formation relatively high
percentages of normal larvae were formed and larvae with non-spatial gaps were
less frequent (Table 2). The constriction apparently was too brief to allow for
cellularization in situ on either side of the constriction and the pinched blastoderm will have returned to its normal position after deconstriction, allowing
re-fusion and the development of normal larvae as late as in stage CB. Moreover,
the briefness of the constriction does not allow the production of large deviations
in the signal distribution which is assumed to be definitive at stage CB. However,
the actual separation of anterior and posterior ooplasm with brief (0-5 min)
constriction in Callosobruchusmzy have lasted longer than the constriction period
proper. In Callosobruchus, the chorion is rather inflexible and often does not
22
J. M. VAN DER MEER
return to its previous shape immediately upon deconstriction, resulting in a
retarded re-fusion. As a result the interference with the distribution of segmental
instructions becomes more extensive and more time is needed for the normalization of this distribution. The time between the postponed re-fusion and segmental commitment, necessary for this normalization, can only be too short with
brief constriction during blastoderm formation (Table 2). This would result in a
discontinuity in the signal distribution, which is read off by the blastoderm cells
and a non-spatial gap would appear. This additional possible mode of nonspatial gap formation is supported by observations in Euscelis and Smittia
(Sander, personal communication). Here, temporary constriction for extended
periods of time also resulted in germ-bands with non-spatial gaps, the yield in
Euscelis corresponding roughly to the yield in Callosobruchus (Table 2).
The origin of multiple segment gaps
When larvae have more than one segment gap, we do not know which gap was
located at the constriction site and which one has to be considered as a remote
constriction effect. We observed eggs with several clefts in the ooplasm near the
constriction region and therefore we believe that they can produce multiple
segment gaps. In addition to the gap at the constriction site, secondary spatial
gaps might occur when larger clefts produce a second complete ooplasmic
separation. Several smaller clefts together might develop into one or two transverse zones resulting in multiple non-spatial gaps.
The absence of segments not situated at the constriction site, particularly in
the non-spatial segment gaps, invalidates the method of characterizing segments
by counting germ-band segments without specific markers. However, in the case
of Callosobruchus larvae, where almost each segment can be recognized with
different markers, this difficulty can be neglected.
Normal larvae from re-fused eggs
Brief temporary constrictions producing re-fused eggs are considered to reveal
whether the absence of segments results from cytological damage. The briefness
of such constrictions excludes an extensive interference with, for instance, the
normal distribution of segmental instructions through the egg, which can be
resumed after re-fusion. Therefore, cytological damage is the only effect left to
explain a possible segment gap.
In Callosobruchus brief constriction in early but not in late stages (Table 2)
confirmed the 95 % normal germ-bands observed after brief temporary constriction at stage SBy/SBy or even later in Euscelis (Armbruster and Sander,
unpublished; cited from Sander, 1975) and similar yields in Smittia (Sander,
1975). The low proportion of normal larvae in the later stages of Callosobruchus
as compared with Euscelis and Smittia may result from differences in egg
characters, like viscosity of the ooplasm or flexibility of the chorion (see above).
Pattern formation in insect embryogenesis
23
ect.
I
(a)
J
(.b)
Fig. 9. Diagrammatic cross sections of normal (a) and premature (b) dorsal closure.
(b) Egg region near the constriction, where the reduced egg space is assumed to result
in premature dorsal closure. Thin arrows: direction of morphogenetic movements;
thick arrows: successive stages of dorsal closure. Abbrev.: a, amnion; ch, chorion;
d, dorsal; ect., ectoderm; mes, mesoderm; s, serosa; v, ventral; y, yolk material.
Explanation: see text.
2. Reduction of egg size by constriction can result in the partial absence of segments
The decrease of the proportion of larvae with dorsal defects from 41 % after
permanent, complete constriction to 18 % after temporary, complete constriction
suggests that they result from the lack of space for normal dorsal closure (Fig.
9 b). In the constriction region the rather stiff chorion is forced down for some
distance on either side of the razor blade. Thus the circumference which can be
closed by dorsal closure decreases. This might result in premature dorsal closure,
whereby a wedge of dorsal embryonic tissue either is not formed or is eliminated.
With temporary constriction the egg circumference in the constriction region is
24
J. M. VAN DER MEER
not so much reduced during dorsal closure, because deconstriction takes place
long before dorsal closure starts.
Because of the absence of only dorsal structures this is not likely to be the
result of interference with the specification of the longitudinal segment pattern;
'ventralized segments' would indicate the presence of the capacity to develop
the complete segment. However, the omission of structures may proceed so far
as to result in the complete disappearance of the segment, even if initially the
instructions for its specification were present. Thus, although the gap phenomenon as such cannot be explained by a lack of egg space, an overestimate of the
gap size might result as an unfortunate side-effect of the constriction technique,
particularly with permanent constriction. The result might be a wrong picture of
the extent of interference of constriction with segment specification.
The dorsal defects allow to discuss an interesting side-question related to the
specification of the transverse cuticular pattern. With decreasing distance from
the constriction region dorsal pattern elements drop out stepwise, starting with
the dorsomedian elements and proceeding bilaterally to the more dorsolateral
elements (Fig. 3 b). Fusion of incompletely extended body flanks might produce
a dorsal anlage which is too small to accommodate a complete left and right
dorsal pattern. The smaller the dorsal anlage, the more dorsomedial parts are
absent. The precise bilaterally symmetrical arrangement of the remaining lateral
parts suggests that the body flanks still possess regulative properties. This is
consistent with the more extensive bilateral regulation observed after cyanide
treatment of Callosobruchus embryos (Brauer, 1938) and with a view of commitment of cells to form a transverse pattern, which proceeds stepwise in time
starting from the ventromedian region of the ventral plate in Tachycines (Krause,
1953, 1962; Krause & Krause, 1957).
3. The extent to which the segment gap can result from cytological damage
We are primarily interested in constriction experiments because we think that
constriction alters the spatial distribution of segmental instructions in the egg
and thus can tell us about how specification occurs. However, it is clear that
constriction can affect the segment pattern by interfering to some extent with the
ability of the cells to complete their programs, i.e. to differentiate. In permanent
complete constrictions this interference occurs directly by the razor blade
separating the two egg halves. If, in temporary constrictions, permanent
cytological damage (cytoplasmic clefts, transverse zones) is left after deconstriction, then this damage can interfere with differentiation. Permanent
constriction cannot distinguish between effects on specification or differentiation
of the segment pattern. Therefore, single and multiple gaps of any type can arise
from interference by the constriction with any of these processes.
The goal of this study was to assess the contribution of cytological damage to
the gap phenomenon. In Callosobruchus a. mosaic mode of metamerization by
prelocalized segment precursors is excluded by the observation of reversed
Pattern formation in insect embryogenesis
25
segment sequences after temporary constriction (Vander Meer, 1978). Therefore,
the question whether constriction produces a segment gap by damage to segment
precursors (Kalthoff, 1976) is not relevant for Callosobruchus and other insects
from which reversal of segment sequence is known.
With permanent, complete constriction only dorsal defects occurred, a
complete absence of segments being hypothetical. The final closure of the gap
with permanent, complete constriction shows that the gap phenomenon can be
ascribed to the effect of the constriction proper and not to the lack of egg space
(Van der Meer and Miyamoto, 1979). However, with permanent, incomplete
and particularly with temporary, complete constriction the cytological damage
remaining after deconstriction can produce a permanent separation of the egg by
one (Fig. 7g, h) or two (Fig. 1 e,f) oolemmata or sheets of blastoderm cells, or by
a transverse zone (Fig. Ik,I). These physiological separations can interfere
with segment specification and/or commitment. Moreover, they may not be as
impermeable to morphogenetic signals as the physical separation by a razor blade
in permanent, complete constrictions (Van der Meer, 1978). On the other hand
we have shown, that in the absence of permanent cytological damage, the
disturbance of the egg system during temporary constriction does not prevent
the development of a normal larva.
The effect of permanent physiological separations on the segment gap can be
studied quantitatively by comparing the gap size of spatial gaps obtained with
permanent, complete constriction with the gap size of spatial and non-spatial
gaps from temporary, complete constrictions. This will be the subject of a future
publication.
I would like to thank Drs S. J. Counce, J. M. Denuce and E. Wieschaus for critical reading
of the manuscript, Dr F. S. Lukoschus for identification of mites and lice. The help of Mr
J. Gerritsen and Mr M. C. J. Groos with the drawings, of Mr J. H. M. Spruyt with the
photographs and of Mrs E. A. J. Derksen with typing the manuscript is gratefully acknowledged. This material is from a thesis submitted for the degree of Doctor of Philosophy in the
Catholic University of Nijmegen, The Netherlands. I owe a special debt to Drs J. Faber and
K. Sander for their help in improving the content and style of my thesis.
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