The 'Dorsal Organ' of Collembolan Embryos.By

0. W. Tiegs

(Department of Zoology, University of Melbourne.)

With Plate 11 and 4 Text-figures.

OUR present knowledge of the problematical' dorsal organ' ofCollembolan 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 ofthe embryos of most Collembola much difficulty has been ex-perienced in making accurate observations on its structure; onlyin the work of Claypole and of Philiptschenko have actualsections been employed for the purpose.

The organ has been found to arise as a thickening of a mediandorsal patch of the blastoderm, the cells of which enlarge andintrude into the underlying yolk. In its deeper portion, i.e. ad-jacent to the yolk, each cell assumes a glandular appearance,while the more peripheral part tapers, and is said to assume afibrillar texture. Eventually, with the appearance of the germ-band, the peripheral part of the organ begins to protrude a littlebeyond the surface of the blastoderm, where it then spreadsoutwards for a short distance, 'like a mushroom'. The pro-truding part of the organ is said to adhere to the overlying partof the blastodermic cuticle, while around the margin of the'mushroom' it is stated to be connected with a second cuticularsheath which invests the embryo, and lies beneath the blasto-dermic cuticle. In the advanced embryo the organ sinks deeperinto 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 thathitherto described for Collembolan embryos; but it differs inthat a long filamentous outgrowth develops from each of itscells, the filaments entering the space between the chorion and

154 O. W. TIEGS

blastodermic cuticle, and growing on to the opposite pole of theegg (Text-fig. 4).

It will be of importance for the theory of the derivation of theInsecta from S y m p h y 1 a-like ancestors if an essentially similarstructure can be demonstrated for the dorsal organ of Collem-bolan embryos.

OBSERVATIONS.

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

Fam. Podur idae .—Hypogas t ru ra a r m a t a (Acho-ru tes armatus) Nic.

Fam. Entomobryidae .—Pseudos ine l la a lba Pack.E n t o m o b r y a m a r g i n a t a

Tlbg.Fam. Sminthur idae .—Sminthurus v i r id i s L.

The eggs of Smin thu rus were kindly sent to me from SouthAustralia by Mr. D. C. Swan; the others were locally procured,and the identification I owe to Mr. H. Womersley of the SouthAustralian Museum.

In the case o f H y p o g a s t r u r a l have had a fairly completeseries of embryos, and most of the critical observations havebeen made on this species; for the others only a few stages ofdevelopment have been available, but these cover that stage inthe formation of the organ on which information is speciallywanted.

A. H y p o g a s t r u r a a r m a t a .

This is a cosmopolitan species, and is one of the formsexamined by Uzel (1898). The eggs are spherical and minute,measuring not more than 0-12 mm. in diameter. They are laidin small clumps in damp situations, and can, with a little prac-tice, be obtained in quantity by scanning with a hand-lens theunder surface of loose stones, wood, or leaves, in places wherethe adults abound. The eggs, thus collected in the field, arefixed in Oarnoy's fluid and preserved in alcohol till required.For staining whole embryos a short immersion in Auerbach's

DORSAL OBGAN OF COLLBMBOLA 155

methyl-green acid-fucfasin mixture has usually given satisfactoryresults, the chorion and, when present, blastodermic cuticlehaving been previously" punctured with a fine needle. For thepreparation of sections eelloidin-embedded material has beenemployed and the sections stained with iron-haematoxylin.

The earliest recognizable stage in the formation of the dorsalorgan is shown in fig. 1, PI. 11. This is from an embryo whichis still at the blastoderm stage of development; the chorion hasnot 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 dorsalorgan are distinguishable from those of the adjacent blastodermby their much greater size and by the paleness of their cyto-plasm ; the nuclei have already begun to retreat from the surface,and in the deeper part of the organ, adjacent to the yolk, thereis just becoming apparent that vacuolation of the cytoplasm,and its capacity for deeper staining, which later characterizesthis zone.

In fig. 2, PI. 11, is shown a more advanced stage in thedevelopment of the organ. This is from an egg in which thesurface distortion of the blastoderm has become more marked.The cells of the dorsal organ have much enlarged and intrudedeeply into the underlying yolk. Owing to encroachment of theadjacent part of the blastoderm, an early stage of which isalready seen in the previous embryo described, the free surfaceof the dorsal organ has become much reduced. In the deeperpart of the organ the vacuolation and rather deeper staining ofthe cytoplasm are now more evident. Towards the periphery, inthe' neck' of the organ, the cells taper; this zone is apt to presenta decidedly fibrillar appearance; but this is not due, as describedby Claypole (1898) and Philiptschenko (1912), to fibrillation ofthe cytoplasm of the cells, but to the convergence of the taperingends of the cells into a gradually narrowing bundle.

Eventually there arises the stage shown in Text-fig. 1, andfig. 3, PL 11. This is from an embryo in which the chorion is stillintact. The surface of the blastoderm is now greatly distorted,and is closely invested by the blastodermic cuticle which investsalso the free surface of the dorsal organ. In the latter the tapering

156 O. W. TIEGS

ends of the cells have begun to protrude a little beyond theblastoderm. When the surface of the organ is closely examinedin the entire egg with an oil-immersion lens, the tapering ends ofthe cells may be seen radiating a short distance away from theorgan (Text-fig. 1). Fig. 3, PI. 11, shows the dorsal organ fromthe same embryo, which has been subsequently sectioned; theribrillar texture of the 'neck' of the organ is again evident, and

0-1 mm.TEXT-FIG. 1.

H y p o g a s t r u r a a r m a t a . An advanced blastoderm at the stageof extreme surface distortion, and prior to the rupture of thechorion. The embryo shows the blunt radiating outgrowths fromthe dorsal organ in an early stage of development. A sectionthrough this embryo is shown in fig. 3, PI. 11.

the individual tapering cells which compose it may be seenbending outwards from the organ, close beneath the blastoder-mic cuticle.

A little later than this the chorion ruptures, and as in certainother Collembola, the blastodermic cuticle takes its place as theprotecting sheath for the embryo. Evidently owing to intake ofmoisture from without, the embryo enlarges, the surface distor-tion 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 blastodermiccuticle (fig. 7, PI. 11; Text-fig. 2 A).

The earliest developmental stage, following upon rupture of

DORSAL ORGAN OF COLLEMBOLA 157

cbc.

B

01mm.

TEXT-FIG. 2.

H y p o g a s t r u r a a r m a t a . Entire embryo, to show dorsal organ.The embryo is an early germ-band; the chorion has already rup-tured, and the blastodermic cuticle has become the functionalegg-shell. The germ-band is still in the dorsally flexed condition;through the provisional body-wall the yolk is still visible (drawnas yolk). The outgrowths from the dorsal organ have now becomelong and filamentous, and spread on to the lower half of the egg.A. seen from left side; B. dorsal view. «, antenna; 6c, blasto-dermic cuticle; c, chorion; Ir, labrum; mn, mandible; mx,maxilla.

NO. 330 M

158 O. W. TIEGS

the chorion, which I have been able to secure, is shown in Text-fig. 2; fig. 4, PL 11, represents a sagittal section along the dorsalwall of an embryo of similar age. The embryo is at the stage inwhich 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 dorsalorgan is an increase in its size, and the markedly greater vacuo-lation which the cytoplasm in the deeper part of the organexhibits. But at the periphery a remarkable change has occurred,the outgrowing ends of the cells having extended as fine fila-ments along the under surface of the blastodermic cuticle, on tothe opposite half of the egg (Text-fig. 2 A). "When the egg isturned so that it can be viewed from above, these filaments areseen radiating outwards from the dorsal organ, spreading on tothe lower half of the egg (Text-fig. 2 B). At their bases, wherethey emerge from the embryo, the outgrowths are, at this stage,still comparatively thick, but thereafter rapidly taper into verydelicate filaments.

There is usually no difficulty in recognizing these filaments attheir point of origin from the dorsal organ; the 'mushroom-shaped' outgrowth shown in the drawings of Claypole and ofPhiliptschenko evidently represents the bases of these developingfilaments, even though the individual filaments were not recog-nized. As the filaments diverge from one another it becomes, inmost whole embryos, increasingly difficult to follow them clearly,for they do not form a ready contrast to the underlying stainedembryo. In sections they are easily recognizable if there is aconspicuous space between the blastodermic cuticle and theembryo; but when the two adhere, the filaments may be verydifficult to detect even in sections.

By the time the appendage-rudiments have become apparentin the germ-band, a second cuticular sheath has developed. It isthinner than the blastodermic cuticle, is closely applied to thesurface of the embryo (fig. 4, PI. 11), and is, for that reason,usually difficult to detect. But in later embryos, after the ven-tral flexure has formed, this cuticle may readily be seen where itstretches from the head to the tip of the abdomen under theventrally flexed embryo (fig. 7, PI. 11), though over most of the

DORSAL ORGAN OF COM.EMBOLA 159

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 secondcuticular sheath has also been described by Claypole (1898)and by Philiptschenko (1912) for the embryos of Anur idaand I s o t o m a respectively; in both these accounts thissecond cuticular sheath is said to become connected with thedorsal organ around the margin of the 'mushroom-shaped' out-growth. Neither in H y p o g a s t r u r a , nor in the embryos ofthe other species which I have examined, is the connexionpresent; instead, the cuticular sheath adheres closely to the wallof the embryo, and can be traced right to the rim of the dorsalorgan (figs. 4, 5, cs2, PI. 11).

In fig. 5, PI. 11, is shown the dorsal organ in a later state ofdevelopment than that depicted in fig. 4, PI. 11. This is froman embryo in which well-developed leg-rudiments are alreadypresent, but in which the ventral flexing of the embryo has notyet begun. The tapering ends of the cells of the dorsal organhave now much diminished in thickness, with the result thatan axial bundle of very delicate filaments is produced, whichshrinks away from the adjacent cells of the dorsal organ. Thedorsal organ thus acquires the form of a hollow cup; the bundleof filaments arises mainly from cells which form the floor of thecup. The second cuticular sheath dips down into the cup. Acomparable stage in the development of the dorsal organ isfigured in Pbiliptschenko's paper on I s o t o m a .

Owing to the close compression of the delicate fibrils whichcomprise the axial bundle of the dorsal organ, the structure ofthis part is difficult to examine. The impression is undoubtedlygiven that the protoplasm of the tapering ends of the cells hasa fibrillar structure, as some authors have described. But, as faras I have been able to observe, the appearance arises purelyfrom the convergence of the rapidly tapering ends of the cells asthey unite to form the axial bundle.

In longitudinal sections of the dorsal organ the individualityof the closely compressed fibrils which comprise the axial bundleis not easily recognizable. But in the transversely cut organ it isat once seen. In figs. 6 A, B, C, PI. 11, are represented three

160 O. W. TIEGS

successive transverse sections through the dorsal organ. In fig.6 A, from near the floor of the organ, the formation of the axialbundle is seen; the polygonal outlines of the tapering ends of thecells, which enter into its formation, are very distinct, and inplaces (viz. to the left of the drawing) the passage of some of thelarge vacuolated cells into the areas, thus defined, is apparent.Pig. 6 B, PL 11, shows a section nearer the surface of the organ;the transversely cut tapering ends of the cells now appear asminute circular areas, closely apposed, though distinct from oneanother. Finally, in fig. 6 c, PL 11, is shown a section cut throughthe orifice of the organ, part of the blastodermie cuticle, with cutedge, and a part of the underlying blastoderm being included inthe section; the divergence of the filaments as they spread out-wards from the dorsal organ is here clearly seen, for they stainwith haematoxylin.

As the ventral flexure of the germ-band develops, the dorsalwall of the embryo becomes much enlarged, the dorsal organtherefore occupying a progressively smaller proportion of thedorsal body-wall (fig. 7, PI. 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 haveformed, degeneration sets in, some of the cells clumping togetherand losing their vacuolation, while, at the same time, the organitself is becoming enclosed within the developing mid-gut (fig. 8,PL 11). In still more advanced embryos the sole remains of thedorsal organ is a clump of disrupting cells lying in the anteriorpart of the cavity of the mid-gut; before the insect emerges thisfinally vanishes.

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

B. P seudos ine l l a a lba .

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

DORSAL ORGAN OF COLLEMBOLA 161

Only a few eggs were obtained, the youngest being in an earlystage of ventral flexure of the germ-band (Text-fig. 3). Thedorsal organ does not differ perceptibly from that of H y p o -g a s t r u r a ; the filamentous outgrowths are recognizable, and

TEXT-FIG. 3.Pseudosinella a lba . Entire embryo, to show dorsal organ.

The embryo is at an early stage of ventral flexure, and is seen fromthe right side. The filaments from the dorsal organ have grown onto the lower pole of the egg. Note the second cuticular sheathstretching across the ventral flexure. The chorion, furnished withlong spines, has not yet ruptured, a. antenna; be, blastodermiccuticle; c, chorion; cs2, second cuticular sheath; Ir, labrum.

may, with care, be traced well on to the ventral half of the egg.They are fewer than i n H y p o g a s t r u r a . As in the latter theyrun on the inner surface of the blastodermic cuticle; the secondblastodermic cuticle is best seen where it stretches across theventral flexure.

In more advanced embryos the remains of the dorsal organ

162 0. W. TIEGS

are to be seen within the lumen of the foregut, and here theyeventually completely disintegrate. The extra-embryonic fila-ments also vanish.

C. E n t o m o b r y a m a r g i n a t a .Only a few advanced eggs of this collembolan have been

secured. They still show remnants of the filaments under theblastodermic cuticle.

D. S m i n t h u r u s v i r i d i s .The eggs of S m i n t h u r u s are spherical and measure about

0-27 mm. in diameter. They are deposited in clumps on moistsoil, and are glued together by a secretion from the anus.Except at early stages of their development the eggs show muchresistance to desiccation. The rupture of the chorion takes placelater than i n H y p o g a s t r u r a . A description of the bionomicsof this insect is given by Davidson (1932).

The chorion is strongly impermeable to fixative. Of severalhundred eggs immersed in Carnoy's fluid only a few, in whichthe chorion had actually ruptured in the fluid, were adequatelyfixed. Better results were given by immersing the eggs in hotCarl's fluid, the heat frequently producing a rupture of thechorion and thereby permitting entry of the fixative.

The material which I have examined covers a series of em-bryos from an early stage of ventral flexure, up to the periodwhere the eyes are already heavily pigmented. The structureof the dorsal organ is similar to that of the other embryosdescribed, and a detailed account is unnecessary. Its relationshipto the cuticular sheaths that invest the embryo also presentsnothing unusual.

GENERAL REMARKS.

From the foregoing account it is evident that the dorsal organof Collembola shows a close resemblance to the organ which Ihave already described under the same name for Symphyla(Tiegs, 1939). The most noteworthy difference is to be found inthe position of the extra-embryonic filaments. In the Symphyla,where the chorion does not undergo precocious rupture, as inmany Collembola, the filaments pass through a hole in the

DORSAL OKGAN OF COLLEMBOLA 163

blastodermic cuticle, the dorsal organ being connected with thiscuticle around the margin of the hole, while the filaments lie inthe space between the cuticle and the chorion. In the Collem-bola, on the contrary, the filaments do not pass through theblastodermic cuticle, but remain in a space bounded externally

be

CHmm.TEXT-FIG. 4.

H a n s e n i e l l a ag i l i s (Symphyla). Entire embryo, from rightside, showing dorsal organ, a, antenna; 6c, blastodermiccuticle; c, chorion; / l t first leg.

by the latter, and internally by the delicate second cuticle. Intheir relation to the dorsal organ, the chorion and blastodermiccuticle of Symphyla evidently stand as the equivalent of theblastodermic cuticle and second cuticular sheath of Collembola,the true chorion of the collembolan egg being often precociouslyshed. But it must be acknowledged that in forms like Smin-t h u r u s, where the chorion remains intact till near the end ofembryonic development, the filaments do not pass through tothe chorion, but remain within a space enclosed externally byblastodermic cuticle, and internally by the second cuticle.

164 O. W. TIEGS

In the entognathous Thysanura dorsal organs have beendescribed—Camp odea (Uzel, 1898), J a p y x (Grassi, 1885);I have not been able to secure any suitable embryos of theseinsects to examine them for the presence of the extra-embryonicfilaments.

The so-called 'dorsal organ' of pterygote insects, althoughoften compared with the organ of similar name from Collembola,probably bears no relation to the latter, for it is a product of thedisruption of the serosa, and not a specific embryonic organ. Incertain pterygote insects, however, the remains of a true dorsalorgan are said to be present—Donacia (Hirschler, 1909),Apis (Nelson, 1915), Sc iara (Du Bois, 1932), Corynodes(Paterson, 1935), and apparently Formica and Chrysomela(Strindberg, 1913). The structure in question is a median dorsalthickening of the blastoderm, which may intrude a little, or beslightly invaginated into, the underlying yolk. It soon dis-appears. Apart from its position in the median dorsal ectodermthe organ shows little in common with the remarkable dorsalorgan of Collembola and Symphyla. Whether it is the vestigeof such an organ we have at present no means of knowing. If itis the homologue of a true dorsal organ, it is surprising thatthere is no reference to its occurrence in Orthopteran embryos.

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

The literature on Crustacean embryology abounds with refer-ences 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 soondegenerate, though in other cases it is apparently glandular.According to Pyatakov the dorsal organ of Argulus plays an

DOKSAL OBGAN OF COLLEMBOLA 165

important role in the shedding of the embryonic cuticle. Thereis nothing in the descriptions hitherto given of these organs inCrustacea, to suggest any resemblance to the dorsal organ ofCollembola and Symphyla.

In the embryo o fLimulus ' dorso-lateral organs' also appear.They exude a glutinous hygroscopic secretion, which appears toaid in the shedding of the embryonic cuticle (Iwanoff, 1933).There is no obvious resemblance to the dorsal organ of Collem-bola and Symphyla.

Erom the foregoing review it would seem that there are, inArthropod embryos, several distinct and phylogenetically un-related organs referred to under the name of 'dorsal organ' or'dorso-lateral organ'. Of these the dorsal organs of Symphylaand Collembola are so similar in structure that an homologybetween the two must probably be conceded; the dorsal organof Entognatha, when carefully examined, will probably be foundto possess a similar structure.

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

Owing to the minuteness of most collembolan embryos it isdifficult to obtain any experimental evidence on this question.In fixed eggs a large space is often seen between the embryo andthe investing blastodermic cuticle, and in such cases the fila-ments may frequently be seen traversing the space, and adheringto the under surface of the cuticle (Text-fig. 3). But in otherembryos no such adhesion to the blastodermic cuticle is to beseen. The space in question is probably the effect of shrinkagefrom the blastodermic cuticle, and even the adhesion of thefilaments to the latter, when it occurs, may be a post-mortemeffect.

The whole structure of the organ, however, points to someentirely different function. Miss Slifer (1938) has suggested thatit may be a water-absorbing organ, analogous to the hydropilediscovered in the embryo of the grasshopper Melanop lus .In Symphyla and Collembola the chorion is readily permeable to

166 O. W. TIBGS

water; this is shown by the ease with which the eggs desiccatein dry air. In Symphyla the structural relationships of the organindeed suggest some such function; the blastodermic cuticle isimpermeable to watery stain (and therefore also probably towater); if the radiating filaments possess some hygroscopicproperty they may perhaps serve as a means whereby water,penetrating through the chorion, is directed to the hole in theblastodermic cuticle, through which the filaments emerge fromthe embryo, and so to the embryo itself.

In some Collembola (Anur ida , I s o t o m a , H y p o g a s -t rura ) there is evidence for a marked imbibition of water, theeggs rapidly swelling at about the time of germ-band formation,with a consequent rupture of the chorion, whose place, as func-tional egg-shell, is then taken by the blastodermic cuticle. Thelatter must then also be permeable to water. If the secondcuticular sheath of collembolan embryos is a water-impermeablemembrane, and if the filaments of the dorsal organ have somehygroscopic property, then the structure of the organ and itsrelation to the cuticles which invest the embryo is at least con-sistent with the conjecture that it may play a part in carryingwater to the tissues of the embryo.

SUMMARY.

1. The cells of the ' dorsal organ' of collembolan embryos giveorigin to long filamentous outgrowths, which extend under thefunctional egg-shell (blastodermic cuticle) well on to the lowerhalf of the egg.

2. In its general structure the organ shows a close resemblanceto the dorsal organ of Symphyla; but there is no good evidencefor any affinity between it and the 'dorsal organs' or 'dorso-lateral organs' that have been described from the embryos ofCrustacea, Arachnida, Chilopoda, or pterygote Insecta.

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

DOBSAL ORGAN OF COLLEMBOLA 167

POSTSCRIPT.Since the present paper was written I have secured the

embryos of three additional species of podurids, representingthree different genera, viz.: Tul lberg ia t i l l y a r d i Worn.,Onychiurus f imetar iusL. , and Achoru tes h i r t e l l u sBorner. For their identification I am indebted to Mr. H.Womersley.

Tul lberg ia t i l l y a r d i is obtainable in abundance inmountainous rain forest country in Victoria. Its eggs, which arewhite and spherical, measure about O12 mm. in diameter, andare laid in small clumps in rotting timber. Onychiurusf i m e t a r i u s i have found locally, living in myriads in a smallpatch of soil rich in humus. Here the eggs are laid in clumps.They are similar, both in appearance and size, to those of thefirst named species. From both I have obtained a large seriesof embryos. Their 'dorsal organs' do not show any markeddifference from those of the other species described in thepresent paper; in particular, the filamentous processes can berecognized, and, with care, followed under the blastodermiccuticle on to the lower half of the egg, where they end. A c h o -ru t e s h i r t e l l u s is one of the large Collembola, and is foundin the same environment as Tul lbergia t i l l y a r d i . Its eggsare 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 dorsalorgan, is exceptionally thick, and from it the delicate filamentsradiate out in enormous numbers under the blastodermic cuticle.I hope to give a fuller description of the dorsal organ in thisspecies in a later paper.

168 O. W. TIEGS

E E F E R B N C B S .

Claypole, A. M., 1898.—' Joum. Morph.', 14.Davidson, J., 1932.—'Australian Joum. Exp. Biol. Med. Sci.', 10.Du Bois, A. M., 1932.—'Joum. Morph.', 54.Grassi, B., 1885.—'Atti dell' Accad. Gioenia di Seienze Nat. Catania',

ser. 3,19.Heymons, B., 1901.—'Zoologies', 33.Hirschler, J., 1909.—'Zeitschr. f. wiss. Zool.', 92.Iwanoff, P. P., 1933.—'Zool. Jahrb. Anat.', 56.Lemoine, V., 1883.—'Assoc. Franc. Congres de la Eochelle.'Manton, S. M., 1928.—'Phil. Trans. Roy. Soc.', B, 216.Nelson, J. A., 1915.—'The embryology of the honey bee', Princeton.Nusbaum, J., and Sohreiber, W., 1898.—'Biol. Centralblatt', 18.Paterson, N. F., 1935.—'Quart. Journ. Micr. Sci.', 78.Philiptschenko, J., 1912.—'Zeitschr. f. wiss. Zool.', 103.Pyatakov, M., 1926.—'Quart. Journ. Mier. Sci.', 70.Slifer, E., 1938.—Ibid., 80.Sollaud, E., 1923.—'Bull. Biol. France et Belg.', Suppl. 5 .Strindberg, H., 1913.—'Zeitschr. f. wiss. Zool.', 106.Tiegs, 0 . W., 1939.—'Quart. Journ. Micr. Sci.', 82.Uljanin, W. N., 1875.—'Bull. Soc. Imp. Am. Sc. Nat. de Moscou', 16.

(Cited from Philiptschenko, 1912.)Uzel, H., 1898.—'Studien iiber die Entwicklung der Apterygoten Insecten.'

Koniggratz.Wheeler, W. M., 1893.—'Journ. Morph.', 8.

EXPLANATION OP PLATE 11.LETTERING.

6, blastoderm; be, blastodermic cuticle; hr, brain; c, chorion; cs2,second cuticular sheath; do, dorsal organ; / , filamentous outgrowths;gc, germ-cells; Ir, labrum; mg, mid-gut epithelium; st, stomodaeum;vng, ventral nerve ganglion.

Fig. 1 .—Hypogas t rura a r m a t a . Section through a fragment ofblastoderm, showing early stage in formation of dorsal organ, which isdistinguishable by the large size and pale staining of its cells. Note distor-tion in adjacent part of blastoderm, x 900.

Fig. 2 . — H y p o g a s t r u r a a r m a t a . The same, in a more advancedstate of development. The 'neck' of the organ is forming by encroachmentof the surrounding blastoderm. Distortion of the blastoderm has increased.The blastodermic cuticle is in course of formation, x 900.

Fig. 3.—H y p o g a s t r u r a a r m a t a . Similar section, from the embryodrawn in Text-fig. 1, i.e. shortly before rupture of chorion. The blastodermic

POBSAL OEGAK OF COLLEMBGLA 169

cuticle is now well defined. The tapering ends of the cells of the dorsalorgan are protruding beyond the surface of the blastoderm, and are begin-ning to bend outwards under the cuticle, x 900.

Fig. 4.—H y p o g a s t r u r a a r m a t a . Sagittal section along the dorsalwall of an embryo at about the stage shown in Text-fig. 2, i.e. after ruptureof the chorion. The tapering ends of the cells of the dorsal organ have begunto grow as filamentous processes along the under surface of the blastodermiccuticle. They are present for only a small part of their length in the section.X900.

Kg. 5.—Hypogastrura a r m a t a . Similar section along the dorsalwall of a more advanced embryo in which legs have already formed, butin which the ventral flexure has not yet appeared. In the 'neck' of theorgan the tapering ends of the cells have become still finer, and now forman axial bundle of very fine fibrillae which are prolonged into filaments thatradiate outwards from the dorsal organ under the blastodermic cuticle.Most of the filaments are cut short at their place of exit from the dorsalorgan; but others can be traced for a considerable distance before they arecut by the microtome knife. X 900.

Fig. 6 A, B, C.—Hypogastrura a r m a t a . Three successive sectionsfrom a transversely cut dorsal organ; from an embryo at about the samestage of development as the last. In 6 A the section passes through thefloor of the dorsal organ, and shows the axial bundle cut near its base. Tothe left the origin of some of its fibres from the cells of the dorsal organ isrecognizable. In 6 B the section passes higher up in the dorsal organ, andshows the axial bundle much diminished in thickness. In 6 c the sectiongrazes along the surface of part of the blastoderm, and some of the overlyingblastodermic cuticle is also present in the section (note its cut edge); thefilaments of the dorsal organ are now seen radiating outwards from itsorifice. X1400.

Fig. 7.—Hypogastrura a r m a t a . Sagittal section along an embryoin which the ventral flexure has appeared, to show position and structureof the dorsal organ. The filaments have all been severed at variable dis-tances after emergence from the orifice of the dorsal organ. X 500.

Fig. 8.—Hypogastrura a rma ta . Anterior end of an embryo insagittal section, showing the dorsal organ becoming enclosed within thedeveloping mid-gut. X 500.

6A

Quart. Journ. Micr. 3d Vol. 83, N.8., PL 11

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