/ . Embryo!, exp. Morph. Vol. 62, pp. 241-258, 1981
Printed in Great Britain © Company of Biologists Limited 1981
241
Formation of the retina-lamina
projection of the cockroach: no evidence for
neuronal specificity
By MARK S. NOWEL
From the Department of Zoology, University of Leicester
SUMMARY
There is a topographical mapping of neural elements onto the lamina neuropile of the
optic lobe of the cockroach, such that adjacent ommatidia project to adjacent points (optic
cartridges) in the lamina neuropile. Postembryonic growth of the compound eye occurs by
addition of new ommatidia to its growing margin. Retinula axons grow from the newly
formed ommatidia to the lamina. By transplantation experiments in which the position or
the orientation of retinal material is altered, it is shown that retinula axons do not make
connections in the lamina with respect to their old position and orientation, but rather, in
keeping with their new situations, apparently maintaining a retinotopic mapping upon the
optic lobe.
INTRODUCTION
Proper functioning of a nervous system depends on the establishment of precise neuronal connections during development. Identifiable cells in the peripheral
nervous system can be shown to establish particular connections with specific
central cells. For example, cereal afferent neurons establish connections with
particular giant axons in the cricket central nervous system (Edwards & Palka,
1974).
Where sense organs consist of repeated identical units (as in the compound
eye), the target may consist of a corresponding set of anatomically identical
neuronal units. In the cockroach compound eye, each ommatidium contains
eight retinula cells. A bundle of eight axons (one axon from each light receptor
cell of the ommatidium) projects from the retina to the lamina, the first synaptic
region in the optic lobe. Here, retinula axons from a single ommatidium
together with a set of five or six second-order cells (plus centrifugal fibres
and other neuronal elements) form the repeated structural units of the
lamina neuropile, the optic cartridges. There is one optic cartridge per ommatidium.
The topographical mapping of the retina upon the lamina requires that
Author's address: Department of Zoology, University of Leicester, Leicester, LEI 7RH,
U.K.
242
M. S. NOWEL
particular first-order retinula cells make connections in particular cartridges.
This paper is concerned with the way in which the retina-lamina projection
arises in the development of the cockroach nervous system, to ask whether or
not differential properties of the neurons are responsible for ordering in the
projection.
According to a neuronal specificity model, retinula axons from ommatidia
in various regions of the compound eye would have characteristic particularities
of some nature which would permit synapses only with neurons having a
complimentary label. As there is a topographical mapping of the retina upon
the lamina in insects (Meinertzhagen, 1976), the neuronal specificity model
requires there to be complimentary labels between neurons in corresponding
regions of the retina and lamina. This model can be tested by transplant
operations which relocate ommatidia in abnormal retinal loci.
Anderson (19786) has suggested that neuronal specificity plays no part in.
the formation of connexions in the retina—lamina projection of another insect
{Schistocerca gregaria). In that study, parts of the developing eye were transplanted heterotopically, and the tracts of nerve fibres which subsequently
developed were examined after silver staining. Those results showed that the
nerve fibre tracts do not appear to grow back to areas of the lamina to which
they would have projected if no operation had been performed. Contact
guidance of growing axons along previously projected fibres more readily
explains her results. Since the claim is an important one, it is essential that
it is substantiated. Describing regenerating tracts of fibres is suggestive, but
only the demonstration of the appropriate pattern of regenerated terminals in
the lamina can provide conclusive proof that connections are not established
by a neuronal specificity mechanism. The purpose of this study is to provide
such proof.
In the present study, a number of experiments to relocate ommatidia have
been performed. By marking sets of graft-derived ommatidia and observing
their retinula axon terminals in the lamina, the establishment of neuronal
connexions between shifted ommatidia and topographically correct regions of
the lamina is now clearly demonstrated. The results agree with Anderson's
(19786) prediction that neuronal specificity does not operate in the establishment of the retina-lamina projection.
MATERIALS AND METHODS
Stocks of P. americana were maintained under standard laboratory conditions
(12 h light and 12 h dark at 24 °C) and fed on rat pellets and water. Surgical
techniques were the same as described previously (Nowel & Shelton, 1980).
Grafts were held in place with a small droplet of melted insect wax (Krogh &
Weis-Fogh, 1951).
Because the retinula axons from transplanted ommatidia degenerate and
Formation of the cockroach retina-lamina projection
243
do not regenerate (Anderson, 1978 a), neuronal specificity had to be tested in
another way. Instead of transplanting differentiated retina, portions of the
growing eye margin (budding zone) were transplanted (Fig. 1). Host animals
were newly-moulted nymphs (larval instars 3-5, or final/penultimate, depending
on the experiment: see below). Grafted integument consisted of the eye margin
(composed of the compound eye proliferation zone), the adjacent vertex
epidermis, and the overlying cuticle. By the time the nymph host had developed
to the adult, a portion of the operated eye was graft-derived and recognizable
by a pigment marker. The stocks used for transplant operations to form these
chimeras were wild type and lavender (Ross, Cochran & Smyth, 1964).
To trace connexion patterns in the optic lobe, small localized lesions were
made in the compound eyes of anaesthetized adult insects. After immobilizing
the cockroach as for a grafting operation (Nowel & Shelton, 1980), a silver
earthing wire was inserted into the head through a hole cut in the cuticle at a
posterior medial point. A tungsten microelectrode was connected to a function
generator set to deliver 2 JLIA at a frequency of 1 MHz. After removal of
narrow strips of the overlying cornea, the microelectrode was inserted into the
exposed ommatidia to be electrolytically destroyed, and left in each point of
insertion for 10 seconds. An ' I ' or ' L ' pattern (the lines approximately five to
eight ommatidia wide) was cauterized in the eye by gradually moving the
microelectrode along the incision. When a particular pattern of microcautery
was completed, the wounds were covered with insect wax. Sixteen hours later
the animal was killed and the eye and optic lobe v/ere fixed in an aldehyde
fixative (Karnovsky, 1965) in a 0-1 M phosphate buffer (Hayat, 1970) at pH 7-4
for 2-4 h. The tissue was post-fixed in phosphate-buffered 1 % osmium tetroxide
for 2-12 h, then dehydrated in an acetone series and cleared in propylene oxide.
Embedding was in Spurr's resin following a long period of infiltration.
The optic lobe was serially sectioned in 1 ju,m sections with a Huxley Ultramicrotome and glass knives, mounted in order on subbed slides, stained with
toluidine blue, and examined with a Zeiss compound microscope. Sections of
lamina neuropile showing degenerating axon terminals (which appear dark
blue following such treatment: Geisert & Altner, 1974) were drawn at a
magnification of 450 x or HOOx using a Zeiss camera lucida. Sections were
also photographed using a Zeiss Photomicroscope II.
RESULTS
Grafting operations on P. americana
Following the exchange of graft material between lavender and wild-type
individuals, tissue which develops from the grafts is recognizable by its distinctive pigmentation (Fig. 2). When the resulting chimeras had reached the
imaginal stage, a pattern was traced in the graft-derived retina with a coagulating
electrode to locally destroy a set of select ommatidia. The position and pattern
244
M. S. NOWEL
(a)
(b)
(c)
(d)
Formation of the cockroach retina-lamina projection
245
of degenerating graft-derived retinula axon terminals in the lamina - the primary
target tissue of the growing retinula projection - was analysed by graphic
reconstruction of the lamina neuropile.
(a) Controls
Lesions were made in the eyes of 14 unoperated wild-type P. americana
adults. The lamina neuropiles were subsequently examined to determine the
regions of retinula axon terminations for sets of (cauterized) ommatidia in
different regions of the compound eye. Results from operated retinae were
compared with those from the unoperated retinae and with control-operated
retinae (those in which a graft was transplanted homotopically).
After approximately 50 % of the experimental and control operations,
cautery in the graft retina results in a complete pattern of retinula axon degeneration in the lamina neuropile. The rest of the lamina appears normal: the cell
body layer appears undamaged, monopolar axons in the lamina neuropile are
healthy, and the size and shape of the neuropile are unaltered. This evidence
argues against the possibility that retinula axon degeneration is caused by direct
injury to the optic lobe. In addition, the anatomical relationship between the
lamina neuropile and that part of the retina receiving the lesion makes the
possibility of direct damage to the lamina extremely unlikely, but does not rule it
out completely. In at least 10 % of the analysed specimens, the retinal degeneration pattern is only partially represented in the lamina. In approximately 40 %
of cases, there is no degeneration detected in the lamina. As retinula axon
degeneration is observed in all of the unoperated animals following retinal
lesions, it is concluded that, where cautery of graft-derived tissue does not
result in degeneration in the lamina, the graft retinula fibres have failed to
reach the lamina. In partial degeneration patterns, it is likely that only part of
the graft projects to the lamina.
Fig. 1. Operations to alter the position or orientation of compound-eye tissue. Squares
of integument including both eye margin and head epidermis were exchanged between
the eyes of newly-moulted wild-type and lavender nymphs of P. americana. (Results
of these operations are shown in Fig. 2.) (a) A-P shift. Grafts from the dorsoposterior region were exchanged with those from the dorsoanterior region to result
in a shift of the graft along the anteroposterior axis. (See Figs 2a, b.) (b) Reversal of
A-P axis. Grafts from right eyes were exchanged with those from left eyes, resulting
in the reversal of the anteroposterior axis. (See Fig. 2c.) (c) 90° rotation. Grafts
including the anterior eye margin were exchanged with l:hose including the dorsal
eye margin, resulting in a 90° rotation of the tissue with respect to both the anteroposterior and the dorsoventral axes. (See Figs Id, e.) id) D-V shift. Grafts including
the dorsal eye margin were exchanged between young nymphs and adults (or
final/penultimate nymphal instars). (See Figs. 2f-h.)
246
M. S. NOWEL
Formation of the cockroach retina-lamina projection
247
(b) Anterior-to-posterior shift
An area of integument including the posterodorsal eye margin was replaced
by a graft including the anterodorsal eye margin (Fig. 1 a). Fourteen resulting
chimeric eyes were analysed (Fig. 2a). Of these, nine animals showed degeneration in the posterior lamina after a lesion was made in the graft-derived retina
(Figs 3 a, 4). No axon degeneration was seen elsewhere in the lamina. Similar
lesions in the posterior portion of two unoperated retinae resulted in degenerating axon terminals in similar regions within the lamina neuropile (Fig. 3 c).
Thus, retinula axons project from anterior ommatidia (shifted to the posterior
part of the compound eye) in keeping with their new positions. They do not
exhibit an 'anterior' specificity.
(c) Posterior-to-anterior shift
In a similar way, posterodorsal eye margin was shifted to an anterodorsal
position (Fig. la). Of 12 chimeras analysed, 9 showed axon degeneration
only in the anterior part of the lamina after a lesion was made in the graftderived retina (Fig. 3 b). The positions of degenerating axons in the lamina
of these operated animals were similar to those found after similar lesions
were made in the two unoperated retinae (Fig. 3 c). Again, shifted
ommatidia project to targets appropriate to their new (rather than their
original) positions.
(d) Anterior-posterior reversal
The dorsal eye margin of a left eye was exchanged with that taken from a
right eye, thus inverting the A-P axis (Fig. 1 b). Of 28 chimeric eyes analysed
(Fig. 2c), 13 showed a pattern of projection in the lamina similar to that in
unoperated animals based on analysis of four control animals (Fig. 5). In
Fig. 2. (a) Chimera of P. americana following an anterior-to-posterior shift of eye
margin between lavender and wild-type nymphs, as in Fig. 1 (a), (b) Chimera produced
by a posterior-to-anterior operation, as in Fig. 1 (a), (c) Chimera produced following
an inversion of the A-P axis, as in Fig. 1 b. Axes of graft an d host tissues are indicated:
A, anterior; D, dorsal, (d) Chimera produced following a clockwise rotation of
90° of the dorsal eye margin, as in Fig. 1 (c). Axes of graft and host tissues are
indicated: A, anterior; D, dorsal, (e) Chimera produced following an anticlockwise
rotation of 90° of the anterior eye margin, as in Fig. 1 c. Axes of graft and host
tissues are indicated: A, anterior; D, dorsal. (/) Chimera produced following a
ventral-to-dorsal shift, as in Fig. Id. (g) A scanning electron micrograph of a
compound eye similar to that shown in Fig. 2/, following a ventral-to-dorsal shift.
Note the raised contour of the grafted material. This may reflect differences in
curvature of the graft and host integument, or differences in adhesive characteristics of host and graft cells resulting in a shortening of the graft/host interface.
g, graft-derived ommatidia; h, host-derived ommatidia. (h) Chimera produced
following a dorsal-to-ventral shift of adult integument, as in Fig. 1 d. Bars represent
0-25 mm.
248
M. S. NOWEL
Experimental
Neuronal
specificity
prediction
(a)
Experimental
Neuronal
specificity
prediction
Formation of the cockroach retina-lamina projection
249
addition, seven animals exhibited a partial projection from the grafted tissue,
though the complete pattern of projection was impossible to determine.
(e) 9.0° rotations
A length of dorsal eye margin was exchanged with a length of anterior eye
margin (Fig. lc). Of 15 chimeric eyes analysed (Figs 2d, e), five showed a
pattern of degeneration similar to that in unoperated animals, based on analysis
of three control animals (Fig. 6).
(/) Ventral-to dorsal shift
A length of dorsal eye margin of a final or penultimate larval instar was
replaced by the dorsal eye margin of a young (third-fifth instar) larva (Fig. 1 d).
Of 14 successful operations, graft ommatidia in four projected to the most
dorsal portion of the lamina only, in keeping with the graft's new dorsal
position in the host eye. It thus compares with the nerve projection from an
unoperated compound eye as determined from the region of axon degeneration
in the lamina following cautery of a group of dorsal ommatidia in the control
eye (Fig. la).
A possibly significant observation is that whereas most grafts are incorporated within the plane of the host eye, the graft-derived ommatidia in the
chimeric eyes formed by a ventral-to-dorsal shift commonly formed an elevated
structure (Fig. 2g). A tenable explanation for this phenomenon is that a
difference in adhesive properties between donor and host tissues causes a
reduction in the extent of the graft/host interface.
Fig. 3. These composite diagrams show the effects of a lesion made in graft-derived
ommatidia following an anterior-to-posterior shift (a), or a posterior-to-anterior
shift (b) (see Fig. 1 a), and can be compared with the effects of similar lesions made in
the anterior (c,) or posterior (cu) portions of control (unoperated) eyes. Diagrams
(fl,) and (bi) show the pattern of degenerating axons (seen in the sectioned lamina as
black dots) following the lesion (heavy line) in the graft portion of the compound
eye (e). A similar set of diagrams (au, 6,,) shows how the pattern of degeneration
would have appeared if the terminals derived from the transplanted retina had
innervated loci of the lamina neuropile appropriate to their pre-operative origin
according to a neuronal specificity model of axon projection, based on results
from control animals (cl5 cu). Sheets of lamina neuropile (1) are shown, with lines
representing the sections which are illustrated alongside. Shading indicates regions
in the lamina where degenerating terminals are actually found (a,, 6, and control
laminae) or where they are expected according to the neuronal specificity model
(aH, />,,). The diagrams show that axons from transplanted pieces of the eye project
to the lamina with no regard for their previous position. Illustrated sections are
camera lucida drawings of sections of the lamina neuropile 20 /*m apart and are
from a complete 1 /wi series. A and P indicate anterior and posterior regions of the
compound eye and lamina.
250
M. S. NOWEL
^
Formation of the cockroach retina-lamina projection
251
(g) Dorsal-to-ventral shift
Dorsal eye margin of a young (third-fifth instar) larva (which would form
central ommatidia in the compound eye of the imago) was replaced by a length
of dorsal eye margin of a newly-emerged adult (which would form the most
dorsal ommatidia of the eye of the imago). Of 20 operations, five were successful
(Fig. 2h). In the two analysed, one had projected to the central region of the
lamina, in keeping with the new position of the graft within the host eye (Fig.
7b), based on results from two control eyes. No axon degeneration was observed
in the lamina of the other animal.
DISCUSSION
In these experiments (Fig. ia-d), the position and/or orientation of pieces
of retina was altered. It is known that these alterations result in permanent
changes in the final pattern of the insect retina. In Schistocerca gregaria,
rotation of the eye margin results in rotation of the polarities of the graftderived ommatidia (Nowel, 1979; P. M. J. Shelton, personal communication).
Furthermore, the 'dorsal spot' (a set of characteristic and obviously different
ommatidia which forms the most dorsal portion of the locust compound eye)
appears at the ventral tip of the eye if the eye margin of the anterodorsal retina
is shifted ventrally (Anderson, 19786; Nowel, 1979). For these reasons it is
certain that graft-derived eye tissue retains its regional determination and
polarity during grafting experiments.
In the developing insect compound eye, as new retinula cells are sequentially
generated and contribute to the formation of new ommatidia, they send out
axons into the developing optic lobe where the outer optic anlage is sequentially
generating ganglion cells. Meinertzhagen (1975) and Shelton (1976) suggest
that the growth phase of these postembryonically developing axons depends
upon contact guidance of a growing axon tracking along a neighbouring
bundle which has grown previously, based on observations of minimal filopodial activity and rapid elongation to the lamina.
On the basis of the present studies, the continued retinotopic projection (as
deduced from the formation of connections between the lamina and eye
tissue in keeping with its transposed rather than original position and/or
orientation) shows that in the postembryonically developing retina-lamina
projection, the matching of specific projecting and target cells (Sperry, 1936)
Fig. 4. Micrographs of semithin sections on which Fig. 3 a is based, showing the
appearance of degenerated axon terminals (arrows) in the lamina neuropile (In).
(a)-(d) correspond to sections nos. 1-4, respectively, in Fig. 3a{. mn, medulla
neuropile; ooa, outer optic anlage; ooc, outer optic chiasma. Bars represent
25 fim.
252
M. S. NOWEL
Neuronalspecificity
prediction
Experimental
Control
Control
(ii)
Fig. 5. These composite diagrams show the effects of a lesion made in graft-derived
ommatidia following an anterior-to-posterior reversal (a) (see Fig. 1 b), and can be
compared with the effects of lesions made in control (unoperated) eyes (b), following
the conventions set out in Fig. 3. The similarities between the pattern of degenerating
terminals in the lamina of the operated animal (a,) and that of the control animal
(bt rather than btt) show that axons from graft-derived ommatidia project to the
lamina with no regard for their pre-operative position or orientation. Sections of
lamina are taken from a complete series and are 20fim apart. A, anterior; P,
posterior; e, compound eye; /, lamina neuropile.
Formation of the cockroach retina-lamina projection
253
does not occur. Rather, the development of the projection which connects the
set of light-receptor cells with the sheet of lamina neuropile can be explained by
contact guidance of nerve fibres along previously projected axons to connect
these two synchronously-growing organs, the retina and the lamina (Anderson,
19786). This would explain both the observations of successful projections from
grafted retinae reimplanted in various orientations or positions, and also many
of the observations of abnormalities in these projections (such as whorls, and
axon tracts parallel to the basement membrane) following operations which
perhaps had damaged the already established nerve pathways (see Anderson,
1976, 19786). This may also explain the occasional observation of patterns of
degenerated axon terminals representing only a portion of the patterns which
had been expected. No firm conclusions could be drawn from the specimens in
question, but even here the incomplete degeneration patterns never supported
a neuronal specificity model. The possibility exists that only a portion of the
graft-derived retinula cells project to the lamina. The other axons may never
reach the lamina neuropile. Rather than following previously-projected retinula
axons to the lamina, axons from the graft may follow misleading pathways as a
result of the disruptive effects of the surgical procedures. Thus, some axons
may be prevented from making appropriate connexions with their target cells
(see Wigglesworth, 1953).
Nardi (1977) notes that following shifts of grafts along the anteroposterior
axis in the retina of Ephestia kuhniella, there is a distortion of the surface
contour of the chimeric eye, with a constriction of the graft-derived ommatidia
into an elevated region. He suggests that there are differences in cell surface
parameters arranged in the form of a gradient along this axis. It has further
been suggested that these differences may specify cell position along at least
one axis of the eye (the A-P axis), and may therefore be involved in the specification of connexions between retina and optic ganglia. Surface molecules on
growing axons may be used to recognize specific target neurons within the brain
(Nardi, 1977). Results similar to those of Nardi (1977) have been found in the
present study (Fig. 2g). Regardless of whether graded cell adhesion variations or
other factors cause the distortion of the graft tissue surface, the present experiments show that in P. americana, axon specification is not correlated with
graft appearance. Axon projection proceeds according to the new position
within the eye whether or not the surface contour of the graft is affected by
transposition along an axis.
These results are in agreement with a number of other studies on insect
nerve development and regeneration which conclude that growing nerve
fibres appear to rely on mechanical guides (such as other nerves) or spatiotemporal cues rather than neuronal specification to reach their target loci. The
establishment of the retina-lamina projection in the imaginal Drosophila is
along the optic stalk which contains the larval ocellar nerve (Hanson, 1972;
Meinertzhagen, 1973, 1975; Trujillo-Cenoz & Melamed, 1973; Melamed &
Q
EMB 62
254
M. S. NOWEL
Neuronal
specificity
prediction
P
Experimental
(b)
(i)
Formation of the cockroach retina-lamina projection
255
Trujillo-Cenoz, 1975). All imaginal discs are connected to the central nervous
system by larval nerves. This may indicate that the mechanical connexion
between target and growing fibres is an important or even indispensible feature
of sensory nerve development, and if these guides are disrupted, other nerves
may be followed centripetally (Ghysen & Deak, 1978). Studies on homeotic
mutants of Drosophila (Stocker, Edwards, Palka & Schubiger, 1976; Stocker,
1977; but see also Vandervorst & Ghysen, 1980), the abdominal sensilla of
Rhodnius prolixus (Wigglesworth, 1953), the antennae of Manduca sexta (Sanes
& Hildebrand, 1975), the 'pioneer neurons' of Locusta legs and antennae
(Bate, 1976), and on cockroach eyes transplanted to the leg (Wolbarsht,
Wagner & Bodenstein, 1966) show developing axons growing along previously
projected nerves. Several studies on regenerating nerves indicate that much
reliance is placed on the presence of a physical bridge at the site of the injury
along which the severed processes can grow (Bodenstein, 1957; Boulton, 1969).
In a number of insect systems, however, specificity best explains the establishment (or re-establishment) of the very accurate projection of relocated or
regenerating nerves to their target sites. Studies on both sensory (Edwards &
Sahota, 1967; Schafer, 1970; Palka & Edwards, 1974; Palka & Schubiger, 1975;
McLean & Edwards, 1976; Anderson & Bacon, 1979) and motoneurons
(Pearson & Bradley, 1972) demonstrate an unequivocal matching of neuron
with target. The consistent patterns of nerve terminals of the locust wing hinge
stretch receptors despite 'mistakes' or 'alternative' courses argue in favour of
the presence of labelled sites in target neuropiles to which the neurons project
(Altman & Tyrer, 1977a, b).
This study has examined the establishment of the topographical projection
of the set of retinula axons on the set of lamina ganglion cells in the cockroach.
From the point of view of the ingrowing retinula fibres, optic cartridges in
different regions of the lamina appear to be equivalent, with no labelled
differences between them. Many questions remain unanswered. Whether or
not the more complex retina-lamina projection patterns found in various
dipterans, for example (Braitenberg, 1967; Zeil, 1979), can be explained by this
sort of mechanism is unknown. In addition, the connections of the neurons
within and between cartridges have not been examined, It is likely that
particular retinula axons within one bundle make selective connexions with
Fig. 6. These composite diagrams show the effects of a lesion made in graft-derived
ommatidia following a 90° rotation (a) (see Fig. 1 c) and can be compared with the
effects of lesions made in control (unoperated) eyes (b), following the conventions
set out in Fig. 3. The similarities between the pattern of degenerating terminals in
the operated animal fa) and that of the control animal (bn rather than bt) show
that axons from graft-derived ommatidia project to the lamina with no regard for
their pre-operative position or orientation. Sections of lamina are taken from a
complete series and those shown are 40fim. apart. A, anterior; P, posterior; e,
compound eye; /, lamina neuropile.
9-2
256
Control
Experimental
(i)
(a)
Neuronal
specificity
prediction
Experimental
(b)
Fig. 7. These composite diagrams show the effects of lesions made in graft-derived
ommatidia following a ventral-to-dorsal shift («,), and a dorsal-to-ventral shift (6,),
and can be compared with the effects of lesions made on control (unoperated)
animals (au), following the conventions set out in Fig. 3. The similarities between the
patterns of degenerating terminals in the operated animal (a,) and the control (fl,,),
and the differences between the operated animal (6,) and that expected according
to the neuronal specificity model (6U) based on the control (flu) show that axons
from graft-derived ommatidia project to the lamina with no regard for their preoperative position. Sections of lamina are taken from a complete series and are
20/tm apart. A, anterior; P, posterior; D, dorsal; V, ventral; e, compound eye;
/, lamina neuropile.
Formation of the cockroach retina-lamina projection
257
the various second order elements which themselves make specific interconnections (Boschek, 1971). The basis of this neuronal specification remains
unknown.
1 am grateful to Dr Peter M. J. Shelton for his help and encouragement throughout this
work, which was supported by a grant to Dr Shelton from the Science Research Council.
I thank Dr M. Ross for the stock of lavender P. americana.
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(Received 22 May 1980, revised 23 October 1980)