ABSTRACT
The developmental behaviour of imaginal discs of Drosophila melanogaster was investigated. The discs were medially halved and transplanted into 55 h old larval hosts to allow a sufficient time for cell proliferation.
The different anlagen for the male first leg are asymmetrically distributed on the leg disc. Its median and lateral halves did not differentiate any structures of the complementary half even after intensive growth. The proliferation resulted only in an enlargement or duplication of the already present anlagen.
In the symmetrical male genital disc certain anlagen could be eliminated by unilateral local irradiation with UV. This technique allowed the production of asymmetrical discs. After cutting, the non-irradiated half formed a complete and symmetrical genital apparatus. The irradiated half differentiated into a symmetrical apparatus which lacked, however, single elements on both sides. A schematic illustration is given.
The results are interpreted to mean that the cell divisions initiated by the fragmentation of the disc always repeat the same state of determination. During a culture period of 3-4 days before metamorphosis a replacement of missing anlagen does not occur.
The two terms ‘regeneration’ and ‘regulation’ are discussed with respect to imaginal discs. We have proposed the modified term ‘proliferative regulation’ to describe the special behaviour of imaginal disc fragments. This phenomenon consists of two processes : (1) an intensive cell proliferation whereby the state of determination is conserved; (2) a division of the overgrown blastema into areas of a fixed size (homonomous arealization).
INTRODUCTION
Earlier experiments have demonstrated a very high ‘regulative’ ability in the genital disc of Drosophila melanogaster. Medially sectioned discs, even from mature larvae, are able to form a fully differentiated normal genital apparatus if the fragments are cultured in vivo in a larval host for a sufficient length of time before metamorphosis. Even a quarter of a disc has this ‘regulative’ ability. However, it should be emphasized that the fragment as a unit is not capable of ‘regulation’, but only its single anlage elements (Hadorn, Bertani & Gallera, 1949; Hadorn, 1963; Ursprung, 1959). These findings appeared to contradict the prevailing belief that Drosophila exhibits a mosaic development. This view was favoured by Geigy (1931), who showed that an anlage pattern of presumptive imaginal structures was already present at an early stage of embryogenesis. Similarly, mosaic development in the leg disc was postulated by Bodenstein (1941), who found, for example, that when half of a leg disc formed the sex comb the complementary half did not.
The unpaired genital disc is a bilaterally symmetrical anlage whose left and right halves contain the same presumptive material (Text-fig. 1,d). In contrast, in the paired leg discs, the anlage material is distributed asymmetrically (Textfig. 2). Thus, medial sectioning of the genital disc results in two equal halves, while any two halves of the leg disc are unequal with respect to their anlage material.
(a) Anlage plan of the male genital disc (Ursprung, 1959). G, Hind gut; A, anal plates; C, claspers; L, lateral plates; PB, peripheral bristles of the genital arch, (b) Procedure for causing unilateral damage in the genital disc by UV-irradiation. S, Plane of sectioning; Co, non-irradiated control.
(a) Anlage plan of the male genital disc (Ursprung, 1959). G, Hind gut; A, anal plates; C, claspers; L, lateral plates; PB, peripheral bristles of the genital arch, (b) Procedure for causing unilateral damage in the genital disc by UV-irradiation. S, Plane of sectioning; Co, non-irradiated control.
Simplified anlage plan of the male foreleg imaginal disc. Part of the prothorax and the different leg segments from the coxa to the tarsus are arranged in ‘concentric’ rings from the outside to the centre. One ring corresponds to one segment whose structural elements are distributed around the ring. During metamorphosis the entire disc is drawn out in a telescopic fashion. M, Median; L, Lateral. Trochanter: dotted area, bristles; hatched area, hairs; 2G St, two groups of sensilla trichodea; Sc+5, five sensilla campaniformia; St 1, one single sensillum trichodeum; EB, edge bristle; Sc−8, eight sens, camp.; Sc 3, three sens, camp.; St 5, row of five sens, trich. Tibia : TR, transversal rows (bristles). First tarsal segment : SC, sex comb. Fifth tarsal segment: Cl, claws. S, Plane of sectioning.
Simplified anlage plan of the male foreleg imaginal disc. Part of the prothorax and the different leg segments from the coxa to the tarsus are arranged in ‘concentric’ rings from the outside to the centre. One ring corresponds to one segment whose structural elements are distributed around the ring. During metamorphosis the entire disc is drawn out in a telescopic fashion. M, Median; L, Lateral. Trochanter: dotted area, bristles; hatched area, hairs; 2G St, two groups of sensilla trichodea; Sc+5, five sensilla campaniformia; St 1, one single sensillum trichodeum; EB, edge bristle; Sc−8, eight sens, camp.; Sc 3, three sens, camp.; St 5, row of five sens, trich. Tibia : TR, transversal rows (bristles). First tarsal segment : SC, sex comb. Fifth tarsal segment: Cl, claws. S, Plane of sectioning.
These facts have led us to ask the following questions. (1) Is it possible to induce regulation and/or regeneration in fragments of young leg discs when transplanted into young larval hosts? (2) What is the behaviour of half of a genital disc which has been made experimentally asymmetrical with respect to one of its anlagen when it has sufficient time for ‘regulative’ growth?
MATERIAL AND METHODS
Drosophila stocks
The wild stock ‘Sevelen’ of Drosophila melanogaster was used in all experiments as larval donor and host. The adult hosts were fertilized females of a homozygous white stock (vv: 1—1·5). The animals were reared on standard food (maize, sugar, agar, yeast) at 25 °C. The age of donors and hosts in each experimental series is given in hours (h) after egg laying.
Technique
Genital discs and first leg discs of male larvae were dissected in Ringer’s solution and cut with a tungsten needle. The fragments were then transplanted into larvae or flies (Ephrussi & Beadle, 1936). We tried to obtain the desired asymmetry of the genital disc by irradiation with a fine UV-beam (Text-fig. 1 >). A 30x50μ2 area was irradiated for 15 sec with a 265 mμ wavelength. The optimal irradiation time must be experimentally determined for each UVsource. A prolonged local irradiation may result in the death of all cells of the imaginal disc. The apparatus used for UV-irradiation was described by Ursprung (1959) who succeeded in obtaining specific defects in the disc. He employed this method to determine the anlage plan of the genital disc.
Experimental plan
(a) First leg disc oj male
Series A : donors, 96 h (late third-instar larvae); hosts, 96 h. This experiment was conducted to determine the anlage plan. For this purpose the discs were cut into several fragments and their developmental capacities were investigated.
Series B: donors, 76-80 h (early third-instar larvae); hosts, 55-58 h (secondinstar larvae). Each half of a disc was implanted into a larval host. The larval hosts were reared individually in separate vials. Thus, it was possible to follow complementary halves of the same disc. This procedure was necessary because the asymmetrical discs could not be cut in exactly the same plane in each case.
(b) Male genital disc
The genital discs were cut in half and transplanted immediately after irradiation into their respective hosts (Text-fig. 1,b). At first the procedure described for series B was used to follow complementary halves of the same disc. Later, this complication was dropped because every median cut resulted in two halves containing portions of all anlagen (Text-fig. 1). Hence any irradiated half is complementary to any non-irradiated half.
Series C: donors, 100 h (late third-instar larvae); hosts, 55-58 h.
Series D: donors, 100 h; hosts, 24 h old adult females.
The fragments were allowed to grow in the adult hosts for 3 days. They were then removed and transplanted into 78 h old larvae where they metamorphosed simultaneously with their hosts. In spite of the additional work involved in using an intermediate adult host, this method was employed because the disc fragments grew sufficiently in the adult milieu so that they could be retransplanted into older larval hosts which have a considerably lower death-rate than young larval hosts. The results of series C and D were the same.
Permanent preparations
We removed the metamorphosed implants from the abdomens of the final hosts 2-3 days after emergence. Whole mounts in Faure’s solution were prepared according to standard procedures. In the genital apparatus only the presence or absence of a certain element was recorded and not the number of bristles on the plates which varied greatly. In some cases the bristle pattern was also altered.
RESULTS
Leg disc
Series A. In order to determine the anlage plan the discs were cut into different fragments and transplanted into mature larvae, where they were forced to begin metamorphosis within a few hours (method described by Hadorn & Buck, 1962). A simplified anlage plan is shown in Text-fig. 2. It contains only those structures which we examined in these experiments. A more detailed anlage plan has been worked out by one of us (G. S.) and will be published elsewhere. Text-fig. 2 also shows where the discs were cut and demonstrates that the anlagen are unequally distributed in the two halves. Each of the halves differentiated only those structures which one would predict from the anlage plan, thus confirming Bodenstein’s results (1941).
Series B. Seventeen complementary halves of single discs were analysed. A typical pair is shown in Plate 1, fig. A. One can see from this illustration that the disc fragments made more structures when they had a prolonged period of growth before pupation (some 70 h), than when they metamorphosed immediately as in series A. But this intensive growth resulted only in a duplication or enlargement of those anlagen already present in the fragment at the time of implantation. No regulation or regeneration of the missing anlagen occurred. Thus, a half disc made either two sex combs or none, depending on whether or not it contained the corresponding anlage material. This was also true for the asymmetrically distributed primordia of the proximal leg segments. Four representative pairs of complementary disc halves are described in Table 1. We observe that the markers under investigation appear, as a rule, in either the median (M) or lateral (L) half, but not in both. The structures whose anlage material is clearly located on the median site (e.g. the transversal rows (TR) of the tibia and the sex comb (SC)) follow this rule without exception. These structures are only differentiated by median halves. The differentiated elements are either double structures or at least contain a greater number of bristles than normal. In some cases we observed that both the median and lateral halves made claws (Plate 1, fig. A and Table 1 pairs 8 and 10). Two other examples of this regeneration-like phenomenon are shown in Table 1. These are the edge bristles (EB) and a group of sensilla (Sc-8) of pairs 10 and 12. These exceptions occurred with the following frequencies: claws 6/17, edge bristles 7/17, sensilla 13/17. These cases will be considered in the discussion.
These results are consistent with the view that the development of leg discs is mosaic in character. This mosaicism manifests itself even in disc fragments which are implanted into young larval hosts where they have a long period of growth before metamorphosis. Under these conditions only a duplication or an en-largement of the anlagen already present is observed.
Genital disc
(a) Preliminary experiments
Control transplantation experiments confirmed that non-irradiated whole male genital discs could make a completely normal sex apparatus (Plate 2, fig. C). In these experiments only the chitinous structures of the external apparatus were dealt with. The inner soft parts—vasa deferentia, paragonia, ductus ejaculatorius, sperm pump and hind gut (Ursprung, 1959)— usually were severely damaged by irradiation and could not be recognized easily. The method of UV-irradiation employed made it possible to eliminate single elements more or less selectively. This was a prerequisite for answering our question as to the behaviour of asymmetrical genital disc halves. We irradiated 30 whole discs and found that none made a normal genital apparatus. In 12 cases all of the structures were present, but either the bristle number was reduced or the topography was disturbed. In the remaining 18 cases we succeeded in completely eliminating one or more elements. The distribution of these eliminations is summarized in Table 2. For our purposes it was not important which anlage elements were missing.
We do not know why both anal plates are often lacking. Even in experiments with non-irradiated genital disc halves the anal plates are the elements most sensitive to injury (Table 4). Heavier doses of UV caused the irradiated area of the normally transparent disc to become opaque and to die. We observed under the microscope that the local irradiation damage spread to an ever increasing area,
We do not know why both anal plates are often lacking. Even in experiments with non-irradiated genital disc halves the anal plates are the elements most sensitive to injury (Table 4). Heavier doses of UV caused the irradiated area of the normally transparent disc to become opaque and to die. We observed under the microscope that the local irradiation damage spread to an ever increasing area,finally causing the death of the entire disc. It may be that even the lower doses induced a wave of damage which resulted in the elimination of both sensitive anal plates. To exclude this factor from our experiments we cut the discs immediately after irradiation.
(b) Series C and D.
We shall present series C and D together since both gave the same results. We found that non-irradiated disc halves could differentiate a complete genital apparatus in most cases (Table 3, row d). Irradiated halves were seldom able to do this. If we take row a and b together we see that nonirradiated half-discs could ‘regulate’ more or less completely in 90-93 % of the cases, while ‘regulation’ took place in irradiated halves in only 20-26 % of the cases. There were instances of special interest in which a symmetrical sex apparatus was made which lacked single elements symmetrically (row e). 55-60% of the irradiated halves belong to this category, in contrast to only 5-7 % for the non-irradiated halves. For our purposes rows a, b, d, and e (Table 3) are the most important ones, since obvious differences between irradiated and nonirradiated half-discs are revealed here. Apparently the irradiation had eliminated single anlage districts which then could no longer be replaced even by intensive growth. This inability resulted in an incomplete genital apparatus with symmetrical defects as illustrated in Plate 2, figs. D, E.
Table 4 shows the distribution of the injuries for the single elements of the genital apparatus. The anal plates are eliminated with the highest frequency and the claspers with the lowest. This differential sensitivity is consistent with the findings in our preliminary experiments. The only structures ever eliminated in non-irradiated discs are the anal plates (Table 3, row e, and Table 4). Only very rarely does the number of plates increase. The central primordia of the penis apparatus are only weakly impaired by the lateral irradiation. The observed defects may also be caused by mechanical injuries induced by cutting the disc medially.
DISCUSSION
Interpretation
A medially halved genital disc, implanted into a mature larval host, forms half of a genital apparatus. Only the central unpaired elements of the penis apparatus become complete through ‘regulative’ growth. However, the same disc fragment differentiates into a whole genital apparatus after transplantation into a young larval host (55-60 h old). In contrast, half of a leg disc can never make a complete leg, even when it is allowed to grow for a long period before metamorphosis (series B). Under such conditions, the disc fragment differentiates a double structure consisting of two symmetrical half legs containing the same elements (Plate 1, fig. A). The complementary structures of the other half of the leg were not made. Hence, when the presumptive anlage material for a structure such as the sex comb is absent (according to our anlage plan), the half-disc is unable to make the missing structure even after intensive growth. This growth results only in a duplication of those structures whose anlagen are already present in the fragment (Table 1). This fact has also be pointed out by Gehring (1966) for fragments of the antennal disc. This mechanism of repetition will simulate production of complementary structures in discs possessing bilateral symmetry such as the genital disc (Text-fig. 1). Here the repetition leads to a restoration of totality, while this is impossible for the asymmetrical leg disc (Text-fig. 2).
The elimination of single elements of the genital disc with UV (Text-fig. 1 ) has shown that the same mechanism of duplication which occurs in fragmented leg discs operates here as well. A genital disc which has lost a particular quality after UV-irradiation cannot restore it. Thus, the genital disc half makes two half-structures, which appear normally as one symmetrical complete genital apparatus. This situation is shown schematically in Text-fig. 3. The genital and leg discs behave in exactly the same way in this respect. However, it should be noticed that when a disc fragment is cultured for a longer period (months and years) it may become able by transdetermination to differentiate structures which were not present before as anlagen in the disc (Hadorn, 1965).
Schematic illustration of the UV-elimination experiment (see Text-fig. 1; Plate 2, figs. D, E; Table 3). I. Male genital disc with anlage regions for lateral plates (L), peripheral bristles (PB), claspers (C), hind gut (G), anal plates (A). S, plane of sectioning. At the right side: elimination of a clasper anlage by UV-irradiation. IL Differentiated structures of the non-irradiated (left side) and irradiated (right side) half-disc. Pe, Penis apparatus. Below this, the result is given in the schematic form used in Table 3.
Schematic illustration of the UV-elimination experiment (see Text-fig. 1; Plate 2, figs. D, E; Table 3). I. Male genital disc with anlage regions for lateral plates (L), peripheral bristles (PB), claspers (C), hind gut (G), anal plates (A). S, plane of sectioning. At the right side: elimination of a clasper anlage by UV-irradiation. IL Differentiated structures of the non-irradiated (left side) and irradiated (right side) half-disc. Pe, Penis apparatus. Below this, the result is given in the schematic form used in Table 3.
The question now arises as to what extent our findings are general. In fact, these results were not unexpected. Multiplication of identical structures and symmetrical duplications have already been demonstrated by Vogt (1946) for eye-antennal disc fragments, by Hadorn & Buck (1962) for wing disc fragments, and most recently by Gehring (1966) for antennal disc fragments. One can see from their results that the fragments never made complementary structures, since complete antennae or wings were never found. An apparent exception to the above is shown in Plate 1, fig. A, where both halves of a leg disc made claws. This could mean that one of the halves regenerated a complementary structure. However, this phenomenon can easily be explained in another way : the cut was made through the middle of the claw anlage, which is thus distributed to both fragments of the disc. Therefore, both halves were able to differentiate claws. This interpretation also holds true for the other examples of this sort listed in Table 1 (EB and Sc-8). Gehring (1966) suggested that the observed differentiation of complementary structures in antennal discs which were cultured for weeks could result from ‘region specific transdetermination ‘. This term refers to a switch in determination from one structure to another whose anlage is normally present in the complete disc. Under this interpretation, the claws, sensilla and edge bristles would arise in complementary disc halves as transdeterminations of cells which were originally determined for other leg structures. However, we believe that this interpretation does not apply to our results since all of the primordia which were made by both halves were located in the middle of the disc where it was cut.
Regeneration and regulation ?
(a) Regeneration
The term ‘regeneration’ denotes those morphogenetic processes which result in a replacement of the missing parts in a structurally and functionally differentiated system (P. Tardent, personal communication). Although imaginal discs responded to the loss of a part with cell divisions, they did not replace missing anlagen in these experiments. They could only duplicate those anlagen which remained. Thus, apart from rare transdetermination (Hadorn, 1965), the dividing cells only repeat their own state of determination. From this we conclude that the term ‘regeneration’ should not be applied to the growth of fragments of imaginal discs, or even to the reconstitution of a single anlage. Although a division of the claw anlage, for example, results in the formation of two claws by a proliferation of the anlage material in question (Plate 1, fig. A), this does not come under the above definition of regeneration, since this growth involved undifferentiated cells. The production of the new cells is not under obvious quantitative control. If a sufficient amount of time is allowed for growth, more material will be made than was originally present. The growth extends even to those regions which are not damaged by the cut. An example of this occurs when a leg disc is cut in half and the edge bristle and the sensilla located on the trochanter away from the wound surface double as a result of proliferation and reorganization of their anlage material (Plate 1, fig. B, EB and Sc∼ 8). The occurrence of duplications of this sort (Table 1) suggests to us that mitoses take place all over the fragment, and not only at the wound surface and in the new blastema as was found by Kroeger (1958) for fragments of wing discs of Ephestia. Whether or not cell migration plays a role in the process of duplication is not known. Hence, it is possible that the cell divisions do not produce a typical blastema, which grows out of the wound, but a general proliferation. These problems are in need of further investigation.
(b) Regulation
The term ‘regulation’ denotes those morphogenetic processes which result in a restitution of wholeness in a structurally and functionally undifferentiated system. This process occurs without an increased rate of cell division and involves only a reorganization of the material already present (P. Tardent, personal communication). At no time do the imaginal discs investigated thus far exhibit regulation as it is defined above. At the most, the behaviour of a single anlage (e.g. for claspers, anal plates, sex comb, etc.) may be considered as regulation (Hadorn et al. 1949; Ursprung, 1959). However, these examples of regulation involve more than a simple reorganization of the material already present, since they also require growth. For example, half of a genital disc contains one clasper anlage; this does not reorganize immediately and form two smaller anlagen, but first enlarges by cell division, preserving the same state of determination. We then suppose that a process of organization occurs (homonomous arealization— Gehring, 1966) which may proceed in the same manner as during normal development.
Regulation requires that the prospective potency be greater than the prospective significance (fate). In agreement with earlier workers we have demonstrated that for all of the structures shown in the anlage plan (Text-figs. 1, 2), the prospective potency is equal to the prospective significance. However, this may not apply to the individual cells of an anlage, since the specific role that a particular cell will have in the formation of a clasper, an anal plate, a sex comb, etc., is probably not determined before metamorphosis. Thus, the prospective potency of single cells may be greater than their prospective significance, but only within the confines of a clasper, an anal plate, a sex comb, etc. Under this interpretation we may speak of this process as regulation. However, since it is preceded by an increase in cell number, we propose to call this special behaviour ‘proliferative regulation’.
Wildermuth & Hadorn (1965) recently found that an intact labial disc is capable of forming a complete proboscis, while its prospective significance is only half a proboscis. Although this appears to be an example of classical regulation, cell divisions are also required here (H. Wildermuth, personal communication). A further example which could be taken as classical regulation was described by Hadorn (1953), who found harmoniously reduced anal plates in fragmented male genital discs of Drosophila séguyi. As LÜônd (1961) revealed in a careful study of subsequent stages of this regulation process, a harmoniously reduced anal plate is formed only when there were more than sixteen bristles, which is half the normal number. When this number was not reached as a result of insufficient time for growth before metamorphosis, the bristle pattern was disproportionate. So here again cell proliferation is necessary for the regulation process. Hence, the term ‘proliferative regulation’ applies to this case as well.
As far as we can see now, proliferative regulation comprises the following processes. (1) A fragmented primordium enlarges by cell multiplication, whereby the area-specific state of determination is reproduced. (2) When the blastema has reached a critical mass at the time of metamorphosis, it is arealized (homonomous arealization) into anlage districts of like size resulting in two or more symmetrical and identical structures. Proliferative regulation is followed by pattern formation in which individual cells are given specific roles (e.g. differentiation into a clasper bristle or a clasper epidermal cell).
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the assistance of Dr H. Oberlander in preparing the English translation of this paper and wish to thank Professor Dr E. Hadorn, Professor Dr P. Tardent and Dr W. Gehring for their critical reading of the manuscript. The publication of this paper was supported by the ‘Karl Hescheler—Stiftung.’
ZUSAMMENFASSUNG
Imaginalscheiben von Drosophila melanogaster wurden halbiert und ihre Entwicklungsleistungen nach Transplantation in junge 55 h alte Larvalwirte untersucht. Die Fragmente erhielten so genfigend Zeit zu ausgedehnter Zellproliferation.
Médiane und latérale Hâlften mannlicher Vorderbeinscheiben, auf der die Anlagen ffir die verschiedenen imaginalen Bildungen des 1. Beines asymmetrisch verteilt sind (Text-fig. 2), differenzierten auch nach intensivem Wachstum keine Strukturen der komplementaren Partnerhâlfte. Die starke Zellproliferation âusserte sich nur in einer Vergrosserung oder Verdoppelung der vorhandenen Anlagen (Plate 1 und Tab. 1).
In der symmetrisch aufgebauten mannlichen Genitalscheibe wurden bestimmte Anlagen mit UV einseitig ausgeschaltet und die Scheibe dadurch experimenten asymmetrisch gemacht (Text-fig. 1). Nach der Halbierung bildete die unbestrahlte Hâlfte einen vollstândigen und symmetrischen Geschlechtsapparat (Plate 2). Die bestrahlte Hâlfte differenzierte ebenfalls einen symmetrischen Geschlechtsapparat, dem aber einzelne Elemente beiderseitig fehlen (Plate 2). Die Versuchsanordnung ist schematisch in Text-fig. 3 dargestellt.
Die Resultate werden dahin interpretiert, dass die Zellteilungen, die im Anschluss an die Fragmentation einsetzen, in der Regel nur gleiche Determinationszustânde repelieren. Etwas Fehlendes wurde unter den gegebenen experimentellen Bedingungen nicht ergânzt.
Die beiden Begriffe ‘Regeneration’ und ‘Regulation’ werden diskutiert und fÜr das spezielle Verhalten der Fragmente von Imaginalscheiben der modifizierte Terminus ‘proliferative Regulation’ vorgeschlagen. Diese umfasst zwei Prozesse: 1. Eine intensive Zellvermehrung, wobei der gleiche Determinationszustand beibehalten wird; 2. Eine Gliederung des Übergrossen Blastems in Areale einer festgesetzten Grosse (homonome Arealisation).
REFERENCES
Fig. A. Structures differentiated by a half leg disc. Al, Median half; A2, Lateral half. TR, transversal rows of the tibia; SC, sex comb; Cl, claws; Tib, tibia; TS, tarsal segment.
Fig. B. Symmetrical duplications in the trochanter differentiated by a median half of a leg disc. EB, Edge bristle; Sc∼8, eight sensilla campaniformia.
Fig. A. Structures differentiated by a half leg disc. Al, Median half; A2, Lateral half. TR, transversal rows of the tibia; SC, sex comb; Cl, claws; Tib, tibia; TS, tarsal segment.
Fig. B. Symmetrical duplications in the trochanter differentiated by a median half of a leg disc. EB, Edge bristle; Sc∼8, eight sensilla campaniformia.
Fig. C. Normal and complete male genital apparatus (only chitinous parts), formed by an implanted untreated whole disc. Pe, Penis apparatus; A, anal plates; C, claspers; L, lateral plates; PB, peripheral bristles of the genital arch.
Figs. D and E. Symmetrical defects in irradiated halves of genital discs. D, Anal plates missing, claspers (C) not yet fully separated. The remaining structures are normal. E, lateral plates missing. The remaining structures are normal, but during handling the topography was altered. Abbreviations are the same as in Fig. C.
Fig. C. Normal and complete male genital apparatus (only chitinous parts), formed by an implanted untreated whole disc. Pe, Penis apparatus; A, anal plates; C, claspers; L, lateral plates; PB, peripheral bristles of the genital arch.
Figs. D and E. Symmetrical defects in irradiated halves of genital discs. D, Anal plates missing, claspers (C) not yet fully separated. The remaining structures are normal. E, lateral plates missing. The remaining structures are normal, but during handling the topography was altered. Abbreviations are the same as in Fig. C.