Our present knowledge of the problematical ‘dorsal organ’ of Collembolan embryos is based mainly on the writings of Uljanin (1875), Lemoine (1883), Wheeler (1893), Claypole (1898), Uzel (1898), and Philiptschenko (1912). Owing to the minute size of the embryos of most Collembola much difficulty has been experienced in making accurate observations on its structure ; only in the work of Claypole and of Philiptschenko have actual sections been employed for the purpose.

The organ has been found to arise as a thickening of a median dorsal patch of the blastoderm, the cells of which enlarge and intrude into the underlying yolk. In its deeper portion, i.e. adjacent to the yolk, each cell assumes a glandular appearance, while the more peripheral part tapers, and is said to assume a fibrillar texture. Eventually, with the appearance of the germband, the peripheral part of the organ begins to protrude a little beyond the surface of the blastoderm, where it then spreads outwards for a short distance, ‘like a mushroom’. The protruding part of the organ is said to adhere to the overlying part of the blastodermic cuticle, w’hile around the margin of the ‘mushroom’ it is stated to be connected with a second cuticular sheath which invests the embryo, and lies beneath the blastodermic cuticle. In the advanced embryo the organ sinks deeper into the yolk, becomes enclosed within the developing mid-gut, and there eventually degenerates.

In the embryos of Symphyla a ‘dorsal organ’ is also present (Tiegs, 1939). It shows the same general structure as that hitherto described for Collembolan embryos; but it differs in that a long filamentous outgrowth develops from each of its cells, the filaments entering the space between the chorion and blastodermic cuticle, and growing on to the opposite pole of the egg (Text-fig. 4).

It will be of importance for the theory of the derivation of the Insecta from Symphyla-like ancestors if an essentially similar structure can be demonstrated for the dorsal organ of Collem- bolan embryos.

The following observations have been made on four species of Collembola, in which the three principal families are represented, as follows:

Fam. Poduridae.—Hypogastrura armata (Achorutes armatus) Nic.

Fam. Entomobryidae.—Pseudosinella alba Pack.

Entomobryamarginata

Tlbg.

Fam. Sminthuridae.—Sminthurus viridis L.

The eggs of Sminthurus were kindly sent to me from South Australia by Mr. D. C. Swan ; the others were locally procured, and the identification I owe to Mr. H. Womersley of the South Australian Museum.

In the case of Hypogastrural have had a fairly complete series of embryos, and most of the critical observations have been made on this species ; for the others only a few stages of development have been available, but these cover that stage in the formation of the organ on which information is specially wanted.

A. Hypogastrura armata

This is a cosmopolitan species, and is one of the forms examined by Uzel (1898). The eggs are spherical and minute, measuring not more than 0·12 mm. in diameter. They are laid in small clumps in damp situations, and can, with a little practice, be obtained in quantity by scanning with a hand-lens the under surface of loose stones, wood, or leaves, in places where the adults abound. The eggs, thus collected in the field, are fixed in Carnoy’s fluid and preserved in alcohol till required. For staining whole embryos a short immersion in Auerbach’s methyl-green acid-fuchsin mixture has usually given satisfactory results, the chorion and, when present, blastodermic cuticle having been previously punctured with a fine needle. For the preparation of sections celloidin-embedded material has been employed and the sections stained with iron-haematoxylin.

The earliest recognizable stage in the formation of the dorsal organ is shown in fig. 1, Pl. 11. This is from an embryo which is still at the blastoderm stage of development ; the chorion has not yet ruptured, but the wrinkling of the. blastoderm surface, already described for the embryos of certain other Collembola (Claypole, Philiptschenko), has begun. The cells of the dorsal organ are distinguishable from those of the adjacent blastoderm by their much greater size and by the paleness of their cytoplasm ; the nuclei have already begun to retreat from the surface, and in the deeper part óf the organ, adjacent to the yolk, there is just becoming apparent that vacuolation of the cytoplasm, and its capacity for deeper staining, which later characterizes this zone.

TEXT-FIG. 1.

Hypogastrura armata. An advanced blastoderm at the stage of extreme surface distortion, and prior to the rupture of the chorion. The embryo shows the blunt radiating outgrowths from the dorsal organ in an early stage of development. A section through this embryo is shown in fig. 3, Pl. 11.

TEXT-FIG. 1.

Hypogastrura armata. An advanced blastoderm at the stage of extreme surface distortion, and prior to the rupture of the chorion. The embryo shows the blunt radiating outgrowths from the dorsal organ in an early stage of development. A section through this embryo is shown in fig. 3, Pl. 11.

In fig. 2, Pl. 11, is shown a more advanced stage in the development of the organ. This is from an egg in which the surface distortion of the blastoderm has become more marked. The cells of the dorsal organ have much enlarged and intrude deeply into the underlying yolk. Owing to encroachment of the adjacent part of the blastoderm, an early stage of which is already seen in the previous embryo described, the free surface of the dorsal organ has become much reduced. In the deeper part of the organ the vacuolation and rather deeper staining of the cytoplasm are now more evident. Towards the periphery, in the ‘neck’ of the organ, the cells taper ; this zone is apt to present a decidedly fibrillar appearance ; but this is not due, as described by Claypole (1898) and Philiptschenko (1912), to fibrillation of the cytoplasm of the cells, but to the convergence of the tapering ends of the cells into a gradually narrowing bundle.

TEXT-FIG. 2.

Hypogastrura armata. Entire embryo, to show dorsal organ. The embryo is an early germ-band; the chorion has already ruptured, and the blastodermic cuticle has become the functional egg-shell. The germ-band is still in the dorsally flexed condition ; through the provisional body-wall the yolk is still visible (drawn as yolk). The outgrowths from the dorsal organ have now become long and filamentous, and spread on to the lower half of the egg. A. seen from left side ; B. dorsal view, a, antenna; bc, blastodermic cuticle; c, chorion; lr, labrum; mn, mandible; mx, maxilla.

TEXT-FIG. 2.

Hypogastrura armata. Entire embryo, to show dorsal organ. The embryo is an early germ-band; the chorion has already ruptured, and the blastodermic cuticle has become the functional egg-shell. The germ-band is still in the dorsally flexed condition ; through the provisional body-wall the yolk is still visible (drawn as yolk). The outgrowths from the dorsal organ have now become long and filamentous, and spread on to the lower half of the egg. A. seen from left side ; B. dorsal view, a, antenna; bc, blastodermic cuticle; c, chorion; lr, labrum; mn, mandible; mx, maxilla.

Eventually there arises the stage shown in Text-fig. 1, and fig. 3, Pl. 11. This is from an embryo in which the chorion is still intact. The surface of the blastoderm is now greatly distorted, and is closely invested by the blastodermic cuticle which invests also the free surface of the dorsal organ. In the latter the tapering ends of the cells have begun to protrude a little beyond the blastoderm. When the surface of the organ is closely examined in the entire egg with an oil-immersion lens, the tapering ends of the cells may be seen radiating a short distance away from the organ (Text-fig. 1). Fig. 3, Pl. 11, shows the dorsal organ from the same embryo, which has been subsequently sectioned; the fibrillar texture of the ‘neck’ of the organ is again evident, and the individual tapering cells which compose it may be seen bending outwards from the organ, close beneath the blastodermic cuticle.

TEXT-FIG. 3.

Pseudosinella alba. Entire embryo, to show dorsal organ. The embryo is at an early stage of ventral flexure, and is seen from the right side. The filaments from the dorsal organ have grown on to the lower pole of the egg. Note the second cuticular sheath stretching across the ventral flexure. The chorion, furnished with long spines, has not yet ruptured, a. antenna ; bc, blastodermic cuticle ; c, chorion ; cs2, second cuticular sheath ; lr, labrum.

TEXT-FIG. 3.

Pseudosinella alba. Entire embryo, to show dorsal organ. The embryo is at an early stage of ventral flexure, and is seen from the right side. The filaments from the dorsal organ have grown on to the lower pole of the egg. Note the second cuticular sheath stretching across the ventral flexure. The chorion, furnished with long spines, has not yet ruptured, a. antenna ; bc, blastodermic cuticle ; c, chorion ; cs2, second cuticular sheath ; lr, labrum.

TEXT-FIG. 4.

Hanse niella agilis (Symphyla). Entire embryo, from right side, showing dorsal organ, a, antenna; bc, blastodermic cuticle; c, chorion; l1 first leg.

TEXT-FIG. 4.

Hanse niella agilis (Symphyla). Entire embryo, from right side, showing dorsal organ, a, antenna; bc, blastodermic cuticle; c, chorion; l1 first leg.

A little later than this the chorion ruptures, and as in certain other Collembola, the blastodermic cuticle takes its place as the protecting sheath for the embryo. Evidently owing to intake of moisture from without, the embryo enlarges, the surface distortion disappears, and the blastodermic cuticle becomes spherical. The egg has now enlarged, measuring 0·17 mm. in diameter. The ruptured chorion adheres in two pieces to the blastodermic cuticle (fig. 7, Pl. 11 ; Text-fig. 2 A).

The earliest developmental stage, following upon rupture of the chorion, which I have been able to secure, is shown in Textfig. 2 ; fig. 4, PI. 11, represents a sagittal section along the dorsal wall of an embryo of similar age. The embryo is at the stage in which the rudiments of the unpaired labrum, of the antennae, mandibles, maxillae, labium, and first leg are already evident. The only perceptible change in the main mass of the dorsal organ is an increase in its size, and the markedly greater vacuolation which the cytoplasm in the deeper part of the organ exhibits. But at the periphery a remarkable change has occurred, the outgrowing ends of the cells having extended as fine filaments along the under surface of the blastodermic cuticle, on to the opposite half of the egg (Text-fig. 2 A). When the egg is turned so that it can be viewed from above, these filaments are seen radiating outwards from the dorsal organ, spreading on to the lower half of the egg (Text-fig. 2 B). At their bases, where they emerge from the embryo, the outgrowths are, at this stage, still comparatively thick, but thereafter rapidly taper into very delicate filaments.

There is usually no difficulty in recognizing these filaments at their point of origin from the dorsal organ; the ‘mushroomshaped’ outgrowth shown in the drawings of Claypole and of Philiptschenko evidently represents the bases of these developing filaments, even though the individual filaments were not recognized. As the filaments diverge from one another it becomes, in most whole embryos, increasingly difficult to follow them clearly, for they do not form a ready contrast to the underlying stained embryo. In sections they are easily recognizable if there is a conspicuous space between the blastodermic cuticle and the embryo ; but when the two adhere, the filaments may be very difficult to detect even in sections.

By the time the appendage-rudiments have become apparent in the germ-band, a second cuticular sheath has developed. It is thinner than the blastodermic cuticle, is closely applied to the surface of the embryo (fig. 4, Pl. 11), and is, for that reason, usually difficult to detect. But in later embryos, after the ventral flexure has formed, this cuticle may readily be seen where it stretches from the head to the tip of the abdomen under the ventrally flexed embryo (fig. 7, Pl. 11), though over most of the surface of the embryo it remains usually very hard to detect. This second cuticle encloses a readily visible coagulable fluid, which occupies the space enclosed by the ventral flexure. A second cuticular sheath has also been described by Claypole (1898) and by Philiptschenko (1912) for the embryos of Anurida and Iso to ma respectively; in both these accounts this second cuticular sheath is said to become connected with the dorsal organ around the margin of the ‘mushroom-shaped’ outgrowth. Neither in Hypogastrura, nor in the embryos of the other species which I have examined, is the connexion present ; instead, the cuticular sheath adheres closely to the wall of the embryo, and can be traced right to the rim of the dorsal organ (figs. 4, 5, cs2, Pl. 11).

In fig. 5, Pl. 11, is shown the dorsal organ in a later state of development than that depicted in fig. 4, Pl. 11. This is from an embryo in which well-developed leg-rudiments are already present, but in which the ventral flexing of the embryo has not yet begun. The tapering ends of the cells of the dorsal organ have now much diminished in thickness, with the result that an axial bundle of very delicate filaments is produced, which shrinks away from the adjacent cells of the dorsal organ. The dorsal organ thus acquires the form of a hollow cup ; the bundle of filaments arises mainly from cells which form the floor of the cup. The second cuticular sheath dips down into the cup. A comparable stage in the development of the dorsal organ is figured in Philiptschenko’s paper onlsotoma.

Owing to the close compression of the delicate fibrils which comprise the axial bundle of the dorsal organ, the structure of this part is difficult to examine. The impression is undoubtedly given that the protoplasm of the tapering ends of the cells has a fibrillar structure, as some authors have described. But, as far as I have been able to observe, the appearance arises purely from the convergence of the rapidly tapering ends of the cells as they unite to form the axial bundle.

In longitudinal sections of the dorsal organ the individuality of the closely compressed fibrils which comprise the axial bundle is not easily recognizable. But in the transversely cut organ it is at once seen. In figs. 6 A, B, C, Pl. 11, are represented three successive transverse sections through the dorsal organ. In fig. 6 A, from near the floor of the organ, the formation of the axial bundle is seen ; the polygonal outlines of the tapering ends of the cells, which enter into its formation, are very distinct, and in places (viz. to the left of the drawing) the passage of some of the large vacuolated cells into the areas, thus defined, is apparent. Fig. 6 B, PL 11, shows a section nearer the surface of the organ; the transversely cut tapering ends of the cells now appear as minute circular areas, closely apposed, though distinct from one another. Finally, in fig. 6 c, Pl. 11, is shown a section cut through the orifice of the organ, part of the blastodermic cuticle, with cut edge, and a part of the underlying blastoderm being included in the section ; the divergence of the filaments as they spread outwards from the dorsal organ is here clearly seen, for they stain with haematoxylin.

As the ventral flexure of the germ-band develops, the dorsal wall of the embryo becomes much enlarged, the dorsal organ therefore occupying a progressively smaller proportion of the dorsal body-wall (fig. 7, Pl. 11). It lies a little behind the head, at about the level of the first thoracic segment.

In this form the organ survives into the advanced embryo. Eventually, at about the time the great nerve-ganglia have formed, degeneration sets in, some of the cells clumping together and losing their vacuolation, while, at the same time, the organ itself is becoming enclosed within the developing mid-gut (fig. 8, Pl. 11). In still more advanced embryos the sole remains of the dorsal organ is a clump of disrupting cells lying in the anterior part of the cavity of the mid-gut ; before the insect emerges this finally vanishes.

After degeneration of the dorsal organ the extraembryonic filaments are no longer to be seen. There is no evidence that they become withdrawn into the embryo itself; they seem, rather, to disintegrate in situ.

B. Pseudosinella alba

The eggs of this collembolan were laid by a batch of insects that had been kept for some time in captivity. The chorion in this species is furnished with long delicate spines.

Only a few eggs were obtained, the youngest being in an early stage of ventral flexure of the germ-band (Text-fig. 3). The dorsal organ does not differ perceptibly from that of Hypo- gastrura; the filamentous outgrowths are recognizable, and may, with care, be traced well on to the ventral half of the egg. They are fewer than inHypogastrura. As in the latter they run on the inner surface of the blastodermic cuticle ; the second blastodermic cuticle is best seen where it stretches across the ventral flexure.

In more advanced embryos the remains of the dorsal organ are to be seen within the lumen of the foregut, and here they eventually completely disintegrate. The extra-embryonic filaments also vanish.

C. Entomobrya marginata

Only a few advanced eggs of this collembolan have been secured. They still show remnants of the filaments under the blastodermic cuticle.

D. Sminthurus viridis

The eggs of Sminthurus are spherical and measure about 0·27 mm. in diameter. They are deposited in clumps on moist soil, and are glued together by a secretion from the anus. Except at early stages of their development the eggs show much resistance to desiccation. The rupture of the chorion takes place later than in Hypogastrura. A description of the bionomics of this insect is given by Davidson (1932).

The chorion is strongly impermeable to fixative. Of several hundred eggs immersed in Carnoy’s fluid only a few, in which the chorion had actually ruptured in the fluid, were adequately fixed. Better results were given by immersing the eggs in hot Carl’s fluid, the heat frequently producing a rupture of the chorion and thereby permitting entry of the fixative.

The material which I have examined covers a series of embryos from an early stage of ventral flexure, up to the period where the eyes are already heavily pigmented. The structure of the dorsal organ is similar to that of the other embryos described, and a detailed account is unnecessary. Its relationship to the cuticular sheaths that invest the embryo also presents nothing unusual.

From the foregoing account it is evident that the dorsal organ of Collembola shows a close resemblance to the organ which I have already described under the same name for Symphyla (Tiegs, 1939). The most noteworthy difference is to be found in the position of the extra-embryonic filaments. In the Symphyla, where the chorion does not undergo precocious rupture, as in many Collembola, the filaments pass through a hole in the blastodermic cuticle, the dorsal organ being connected with this cuticle around the margin of the hole, while the filaments lie in the space between the cuticle and the chorion. In the Collembola, on the contrary, the filaments do not pass through the blastodermic cuticle, but remain in a space bounded externally by the latter, and internally by the delicate second cuticle. In their relation to the dorsal organ, the chorion and blastodermic cuticle of Symphyla evidently stand as the equivalent of the blastodermic cuticle and second cuticular sheath of Collembola, the true chorion of the collembolan egg being often precociously shed. But it must be acknowledged that in forms like Smin- thurus, where the chorion remains intact till near the end of embryonic development, the filaments do not pass through to the chorion, but remain within a space enclosed externally by blastodermic cuticle, and internally by the second cuticle.

In the entognathous Thysanura dorsal organs have been described—Camp odea (Uzel, 1898), Japyx (Grassi, 1885); I have not been able to secure any suitable embryos of these insects to examine them for the presence of the extra-embryonic filaments.

The so-called ‘dorsal organ’ of pterygote insects, although often compared with the organ of similar name from Collembola, probably bears no relation to the latter, for it is a product of the disruption of the serosa, and not a specific embryonic organ. In certain pterygote insects, however, the remains of a true dorsal organ are said to be present—Dona ci a (Hirschler, 1909), Apis (Nelson, 1915), Sciara (Du Bois, 1932), Corynodes (Paterson, 1935), and apparently Formica and Chrysomela (Strindberg, 1913). The structure in question is a median dorsal thickening of the blastoderm, which may intrude a little, or be slightly invaginated into, the underlying yolk. It soon disappears. Apart from its position in the median dorsal ectoderm the organ shows little in common with the remarkable dorsal organ of Collembola and Symphyla. Whether it is the vestige of such an organ we have at present no means of knowing. If it is the homologue of a true dorsal organ, it is surprising that there is no reference to its occurrence in Orthopteran embryos.

There does not seem to be any record of a dorsal organ from the embryos of Diplopoda. Amongst the Chilopoda, however, Heymons (1901) has described, under this name, a conspicuous crescentic thickening of the ectoderm, several cells in depth, lying a little behind the head, whose cells soon undergo disruption. It shows, however, so little resemblance in its structure to the dorsal organ of Collembola and Symphyla, that any relation between the two must remain doubtful.

The literature on Crustacean embryology abounds with references to ‘dorsal’ and paired ‘dorso-lateral’ organs—cf. Nus- baum and Schreiber (1898), Sollaud (1923), Manton (1928), Pyatakov (1926), and many others. The ‘dorso-lateral organs’ appear to be glandular structures ; the unpaired ‘dorsal organ’, in some cases at least, comprises clumps of cells which soon degenerate, though in other cases it is apparently glandular. According to Pyatakov the dorsal organ of Argulus plays an important rôle in the shedding of the embryonic cuticle. There is nothing in the descriptions hitherto given of these organs in Crustacea, to suggest any resemblance to the dorsal organ of Collembola and Symphyla.

In the embryo ofLimulus1 dorso-lateral organs’ also appear. They exude a glutinous hygroscopic secretion, which appears to aid in the shedding of the embryonic cuticle (Iwanoff, 1933). There is no obvious resemblance to the dorsal organ of Collembola and Symphyla.

From the foregoing review it would seem that there are, in Arthropod embryos, several distinct and phylogenetically unrelated organs referred to under the name of ‘dorsal organ’ or ‘dorso-lateral organ’. Of these the dorsal organs of Symphyla and Collembola are so similar in structure that an homology between the two must probably be conceded ; the dorsal organ of Entognatha, when carefully examined, will probably be found to possess a similar structure.

The function of the dorsal organ is at present obscure. The texture of the cytoplasm of its cells suggested to Philiptschenko (1912) some excretory or secretory function, and according to this observer it was also a means whereby the embryo attached itself to the investing cuticle.

Owing to the minuteness of most collembolan embryos it is difficult to obtain any experimental evidence on this question. In fixed eggs a large space is often seen between the embryo and the investing blastodermic cuticle, and in such cases the filaments may frequently be seen traversing the space, and adhering to the under surface of the cuticle (Text-fig. 3). But in other embryos no such adhesion to the blastodermic cuticle is to be seen. The space in question is probably the effect of shrinkage from the blastodermic cuticle, and even the adhesion of the filaments to the latter, when it occurs, may be a post-mortem effect.

The whole structure of the organ, however, points to some entirely different function. Miss Slifer (1938) has suggested that it may be a water-absorbing organ, analogous to the hydropile discovered in the embryo of the grasshopper Melanoplus. In Symphyla and Collembola the chorion is readily permeable to water ; this is shown by the ease with which the eggs desiccate in dry air. In Symphyla the structural relationships of the organ indeed suggest some such function ; the blastodermic cuticle is impermeable to watery stain (and therefore also probably to water); if the radiating filaments possess some hygroscopic property they may perhaps serve as a means whereby water, penetrating through the chorion, is directed to the hole in the blastodermic cuticle, through which the filaments emerge from the embryo, and so to the embryo itself.

In some Collembola (Anurida, Isotoma, Hypogas- trura) there is evidence for a marked imbibition of water, the eggs rapidly swelling at about the time of germ-band formation, with a consequent rupture of the chorion, whose place, as functional egg-shell, is then taken by the blastodermic cuticle. The latter must then also be permeable to water. If the second cuticular sheath of collembolan embryos is a water-impermeable membrane, and if the filaments of the dorsal organ have some hygroscopic property, then the structure of the organ and its relation to the cuticles which invest the embryo is at least consistent with the conjecture that it may play a part in carrying water to the tissues of the embryo.

  1. The cells of the ‘dorsal organ’ of collembolan embryos give origin to long filamentous outgrowths, which extend under the functional egg-shell (blastodermic cuticle) well on to the lower half of the egg.

  2. In its general structure the organ shows a close resemblance to the dorsal organ of Symphyla ; but there is no good evidence for any affinity between it and the ‘dorsal organs’ or ‘dorsolateral organs’ that have been described from the embryos of Crustacea, Arachnida, Chilopoda, or pterygote Insecta.

  3. While its function is unknown, its structural relations are at least consistent with the view that it may be an organ for the transference of water from without to the embryonic tissues.

Since the present paper was written I have secured the embryos of three additional species of podurids, representing three different genera, viz.: Tullbergia tillyardi Worn., Onychiurus fimetarius L., and Achorutes hirtellus Börner. For their identification I am indebted to Mr. H. Womersley.

Tullbergia tillyardi is obtainable in abundance in mountainous rain forest country in Victoria. Its eggs, which are white and spherical, measure about 0·12 mm. in diameter, and are laid in small clumps in rotting timber. Onychiurus fimetarius I have found locally, living in myriads in a small patch of soil rich in humus. Here the eggs are laid in clumps. They are similar, both in appearance and size, to those of the first named species. From both I have obtained a large series of embryos. Their ‘dorsal organs’ do not show any marked difference from those of the other species described in the present paper; in particular, the filamentous processes can be recognized, and, with care, followed under the blastodermic cuticle on to the lower half of the egg, where they end. A ch o - rutes hirtellus is one of the large Collembola, and is found in the same environment as Tullbergia tillyardi. Its eggs are laid in clumps and measure about 0·42 mm. in diameter. The ‘dorsal organ’ in this species is magnificently developed; the central core of filaments, where these emerge from the dorsal organ, is exceptionally thick, and from it the delicate filaments radiate out in enormous numbers under the blastodermic cuticle. I hope to give a fuller description of the dorsal organ in this species in a later paper.

1.
Claypole
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A. M.
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1898
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‘Joum. Morph.’
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14
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2.
Davidson
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J.
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‘Australian Joum. Exp. Biol. Med. Sci.’
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10
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Du Bois
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A. M.
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1932
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‘Joum. Morph.’
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54
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Grassi
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B.
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1885
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‘Atti dell’ Accad. Gioenia di Scienze Nat. Catania’, ser. 3
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19
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Heymons
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R.
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1901
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‘Zoológica’
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33
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Hirschler
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J.
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1909
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‘Zeitsehr. f. wiss. Zool.’
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92
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7.
Iwanoff
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P. P.
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1933
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‘Zool. Jahrb. Anat.’
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56
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8.
Lemoine
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V.
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1883
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‘Assoc. Franç. Congrès de la Rochelle.’
9.
Manton
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S. M.
,
1928
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‘Phil. Trans. Roy. Soc.’, B
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216
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10.
Nelson
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J. A.
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1915
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‘The embryology of the honey bee’
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Princeton
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11.
Nusbaum
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J.
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Schreiber
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W.
,
1898
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‘Biol. Centralblatt’
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18
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12.
Paterson
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N. F.
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1935
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‘Quart. Joum. Micr. Sci.’
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78
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13.
Philiptschenko
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J.
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1912
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‘Zeitsehr. f. wise. Zool.’
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103
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14.
Pyatakov
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M.
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1926
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‘Quart. Joum. Micr. Sci.’
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Slifer
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E.
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1938
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—Ibid
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80
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Sollaud
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E.
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1923
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‘Bull. Biol. France et Belg.’, Suppl
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5
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Strindberg
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Tiegs
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1939
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Uljanin
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Uzel
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8
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PLATE 11.

LETTERING.

b, blastoderm; bc, blastodermic cuticle; br, brain; c, chorion; cs2, second cuticular sheath; do, dorsal organ; f, filamentous outgrowths; gc, germ-cells; lr, labrum; mg, mid-gut epithelium; st, stomodaeum; mg, ventral nerve ganglion.

Fig. 1.—Hypogastrura armata. Section through a fragment of blastoderm, showing early stage in formation of dorsal organ, which is distinguishable by the large size and pale staining of its cells. Note distortion in adjacent part of blastoderm, × 900.

Fig. 2.—Hypogastrura armata. The same, in a more advanced state of development. The ‘neck’ of the organ is forming by encroachment of the surrounding blastoderm. Distortion of the blastoderm has increased. The blastodermic cuticle is in course of formation, × 900.

Fig. 3.—Hypogastrura armata. Similar section, from the embryo drawn in Text-fig. 1, i.e. shortly before rupture of chorion. The blastodermic cuticle is now well defined. The tapering ends of the cells of the dorsal organ are protruding beyond the surface of the blastoderm, and are beginning to bend outwards under the cuticle, × 900.

Fig. 4.—H ypogastruraarmata. Sagittal section along the dorsal wall of an embryo at about the stage shown in Text-fig. 2, i.e. after rupture of the chorion. The tapering ends of the cells of the dorsal organ have begun to grow as filamentous processes along the under surface of the blastodermic cuticle. They are present for only a small part of their length in the section. × 900.

Fig. 5.—Hypogastrura armata. Similar section along the dorsal wall of a more advanced embryo in which legs have already formed, but in which the ventral flexure has not yet appeared. In the ‘neck’ of the organ the tapering ends of the cells have become still finer, and now form an axial bundle of very fine fibrillae which are prolonged into filaments that radiate outwards from the dorsal organ under the blastodermic cuticle. Most of the filaments are cut short at their place of exit from the dorsal organ ; but others can be traced for a considerable distance before they are cut by the microtome knife. × 900.

Fig. 6 A, B, C.—Hypogastrura armata. Three successive sections from a transversely cut dorsal organ ; from an embryo at about the same stage of development as the last. In 6 A the section passes through the floor of the dorsal organ, and shows the axial bundle cut near its base. To the left the origin of some of its fibres from the cells of the dorsal organ is recognizable. In 6 B the section passes higher up in the dorsal organ, and shows the axial bundle much diminished in thickness. In 6 c the section grazes along the surface of part of the blastoderm, and some of the overlying blastodermic cuticle is also present in the section (note its cut edge) ; the filaments of the dorsal organ are now seen radiating outwards from its orifice. ×1400.

Fig. 7.—Hypogastrura armata. Sagittal section along an embryo in which the ventral flexure has appeared, to show position and structure of the dorsal organ. The filaments have all been severed at variable distances after emergence from the orifice of the dorsal organ. × 500.

Fig. 8.—Hypogastrura armata. Anterior end of an embiyo in sagittal section, showing the dorsal organ becoming enclosed within the developing mid-gut. × 500.

PLATE 11.

LETTERING.

b, blastoderm; bc, blastodermic cuticle; br, brain; c, chorion; cs2, second cuticular sheath; do, dorsal organ; f, filamentous outgrowths; gc, germ-cells; lr, labrum; mg, mid-gut epithelium; st, stomodaeum; mg, ventral nerve ganglion.

Fig. 1.—Hypogastrura armata. Section through a fragment of blastoderm, showing early stage in formation of dorsal organ, which is distinguishable by the large size and pale staining of its cells. Note distortion in adjacent part of blastoderm, × 900.

Fig. 2.—Hypogastrura armata. The same, in a more advanced state of development. The ‘neck’ of the organ is forming by encroachment of the surrounding blastoderm. Distortion of the blastoderm has increased. The blastodermic cuticle is in course of formation, × 900.

Fig. 3.—Hypogastrura armata. Similar section, from the embryo drawn in Text-fig. 1, i.e. shortly before rupture of chorion. The blastodermic cuticle is now well defined. The tapering ends of the cells of the dorsal organ are protruding beyond the surface of the blastoderm, and are beginning to bend outwards under the cuticle, × 900.

Fig. 4.—H ypogastruraarmata. Sagittal section along the dorsal wall of an embryo at about the stage shown in Text-fig. 2, i.e. after rupture of the chorion. The tapering ends of the cells of the dorsal organ have begun to grow as filamentous processes along the under surface of the blastodermic cuticle. They are present for only a small part of their length in the section. × 900.

Fig. 5.—Hypogastrura armata. Similar section along the dorsal wall of a more advanced embryo in which legs have already formed, but in which the ventral flexure has not yet appeared. In the ‘neck’ of the organ the tapering ends of the cells have become still finer, and now form an axial bundle of very fine fibrillae which are prolonged into filaments that radiate outwards from the dorsal organ under the blastodermic cuticle. Most of the filaments are cut short at their place of exit from the dorsal organ ; but others can be traced for a considerable distance before they are cut by the microtome knife. × 900.

Fig. 6 A, B, C.—Hypogastrura armata. Three successive sections from a transversely cut dorsal organ ; from an embryo at about the same stage of development as the last. In 6 A the section passes through the floor of the dorsal organ, and shows the axial bundle cut near its base. To the left the origin of some of its fibres from the cells of the dorsal organ is recognizable. In 6 B the section passes higher up in the dorsal organ, and shows the axial bundle much diminished in thickness. In 6 c the section grazes along the surface of part of the blastoderm, and some of the overlying blastodermic cuticle is also present in the section (note its cut edge) ; the filaments of the dorsal organ are now seen radiating outwards from its orifice. ×1400.

Fig. 7.—Hypogastrura armata. Sagittal section along an embryo in which the ventral flexure has appeared, to show position and structure of the dorsal organ. The filaments have all been severed at variable distances after emergence from the orifice of the dorsal organ. × 500.

Fig. 8.—Hypogastrura armata. Anterior end of an embiyo in sagittal section, showing the dorsal organ becoming enclosed within the developing mid-gut. × 500.