The Development of the Alimentary Canal in Fieris Brassicae and the Endodermal origin of the Malpighian Tubules of Inseots.

Transcription

1 The Development of the Alimentary Canal in Fieris Brassicae and the Endodermal origin of the Malpighian Tubules of Inseots. By H. Henson, B.Sc, Ph.D., Assistant Lecturer in Zoology in the University of Leeds. With Plate 18 and 9 Text-figures. CONTENTS. PAGE 1. INTRODUCTION INTERPRETATION OF THE INTERSTITIAL RINGS RELATION OF THE ATTACHMENT OF THE MALFIGHIAN TUBULES TO THE INTERSTITIAL RINGS DEVELOPMENT OF THE ALIMENTARY CANAL IN PIERIS (1) Cleavage and Gastrulation (2) The fore-gut 291 (3) The hind-gut SUMMARY BIBLIOGRAPHY EXPLANATION OF PLATE INTRODUCTION. IN a previous paper I have attempted to give a detailed account of the structure of the alimentary canal in Vanessa urticae (Henson, 1931). During the course of that work it became clear that accurate morphological definitions of the various parts of the gut would only be possible after a detailed study of its embryology. The structure of the larval Malpighian tubules had indicated that, in spite of their position on the hind-gut and their apparent derivation from the proctodaeum, they were not necessarily ectodermal and were indeed more likely to be endodermal derivatives. Their development was thus of considerable significance in relation to general embryological interpretation. My best thanks are due to Prof. W. Garstang and Prof. P. Balfour Browne for their kindly encouragement and criticism.

2 284 H. HENSON 2. INTERPRETATION OP THE INTERSTITIAL EINGS. In the previous paper cited above it has been shown that in the larval alimentary canal there are two rings of cells which are persistently embryonic throughout life. One, the anterior interstitial ring, lies between the fore-gut and mid-gut, and the other, the posterior interstitial ring, between the mid-gut and hind-gut. Their presence renders the exact boundary between the mid-gut and the stomodaeal and proctodaeal parts of the alimentary canal rather indefinite (vide Text-fig. 7, and figs. 6 and 9, PI. 14, loc. cit.). I expressed the opinion that they were the persistent embryonic ends of the stomodaeum and proctodaeum which grew inwards in the embryo by a kind of terminal meristem. Subsequent investigation of embryos has shown that this idea is untenable and some other conception is necessary. If we attempt to define the interstitial rings we should have to say that they are regions where the ectoderm of the stomodaeum and proetodaeum runs indistinguishably into the endoderm of the mesenteron. This type of definition is applicable to the lips of the blastopore in many animal embryos, and shows that the correct interpretation of these rings may involve the general theory of gastrulation in insects. Sedgwick (1885) has shown that the blastopore in Peripatus capensis divides into two parts by closure of its middle region. The two entrances into the gut thus left are spoken of by him as embryonic mouth and anus. These are subsequently carried inwards by the ingrowing stomodaeum and proetodaeum but are never closed. The relationship of the germ-layers before and after this process and the position of the blastopore lips are indicated in Text-fig. 1. It will be seen at once that the blastopore lips are exactly in the position of the interstitial rings of the insect alimentary canal. The development of the interstitial rings in the embryo insect should thus furnish an invaluable guide to the exact position of the blastopore lips. In no embryo, however, has it been possible to see any histological differences between those cells which become the interstitial rings and those of the rest of the stomodaeum and proetodaeum. That there is some

3 DEVELOPMENT OF PIEEIS 285 essential difference is only revealed by their failure to differentiate in post-embryonic phases. TEXT-FIG. 1. Gastrulation in P e r i p a t u s capensis illustrating the relationships of the blastopore lips to the interstitial (imaginal) rings of insects. A, B, and C, diagrams of surface views of a Peripatus.gastrula showing the closure of the middle part of the blastopore. D, a longitudinal section of a gastrula before the closure. E, a longitudinal section after closure. F, a longitudinal section of a Peripatus embryo after the formation of the stomodaeum and proctodaeum. G, a generalized longitudinal section of a caterpillar for comparison with F. b, blastopore; bl I, cells of the blastopore lip; e a, embryonic anus; ect, ectoderm; e m, embryonic mouth; end, endoderm; ini ring, interstitial ring; mes, mesenteron; pr, proctodaeum; st, stomodaeum. The ingrowths from the germ-band usually spoken of as stomodaeum and proctodaeum might not be purely ectodermal in insects. As will be shown later their blind ends develop into

4 286 H. HBNSON quite definite mid-gut cells. The rim immediately adjacent to the blind end becomes in each case the interstitial ring. I therefore suggest that the so-called proctodaeal and stomodaeal ingrowths in insects are complex structures composed of the proctodaeum and stomodaeum proper, those portions of the blastopore lips corresponding to the embryonic mouth and anus of Peripatus, and a portion of true endoderm internally. 3. RELATION OF THE ATTACHMENT OF THE MALPIGHIAN TUBULES TO THE INTERSTITIAL EINGS. In examining the literature with regard to the development of Malpighian tubules I have found nothing beyond the statement that they arise as outgrowths from the proctodaeum. On this evidence embryologists appear to regard them as ectodermal derivatives. The viewpoint depends entirely on the assumption that the proctodaeal ingrowth is proetodaeum and nothing else. If, however, the blind end of the proctodaeum in insects contains both endoderm and the homologue of blastopore lips the view is open to question. The situation is very similar to that with regard to the mid-gut. Many authors, Mansour (1927), Leuzinger, &c. (1926), have regarded the mesenteron as ectodermal because it arises in association with the ends of the stomodaeum and proctodaeum. For a full discussion of this subject see Eastham's Review (1930). The organs we know as Malpighian tubules have been found in Amphipod Crustacea, certain Arachnids, and Insects. They have long since been recognized as mesenteron appendages in the Crustacea and Arachnida; it is only in the insects that an ectodermal derivation from the proctodaeum has been claimed for them. This is rather curious in view of the fact that in many insects they are distinctly attached to the mesenteron and not to the hind-gut. In the past it has been customary to deny the homology of the Malpighian tubules in the three groups. I believe they are homologous and endodermal throughout. As regards the Amphipod Crustacea they have been described in Melita, Corophium, Gammarus, Orchestia, Talitrus, &c. Baldwin Spencer (1885) has given an account

5 DEVELOPMENT OF PIERIS 287 of their anatomy in Gammarus and Ta 1 itr us. They quite definitely open into the posterior end of the mid-gut. Pereyaslawzewa (1888 quoted from Korschelt and Heider) describes them as arising in the embryo as mid-gut diverticula. Baldwin Spencer concluded that they were indisputably endodermal and therefore not homologous with the tubules of insects. I prefer to regard their well-established endodermal nature in this group as an indication of their primarily endodermal nature throughout the Arthropods. In Lithobius there are two Malpighian tubules, one on each side of the body, opening into the hind end of the midgut (fig. 1, PI. 18). The proximal end of the tubule is expanded to form an ampulla. The position is exactly like that described above for the Amphipod Crustacea. In insects the attachment of the tubules seems to vary. In some groups they open into the mid-gut, in others into the hindgut. Only a few examples will be given here. Davis (1927) writing of Stenopelmatus (Orthoptera) describes six ureters (i.e. proximal ampullae receiving the tubules proper) lined with the kind of epithelium typical of the mid-intestine, and opening into the posterior end of this organ. The morphology seems to be quite comparable with that of Lithobius and the Crustacea except for the increased number of tubules. I have personally verified the fact that even in a specialized form like C a 11 i p h o r a (Diptera) the tubules have no association with the hind-gut (vide also Perez, 1910). In the Lepidoptera the morphology of the attachrnent of the tubules is extremely interesting and illuminating. In H e p i a 1 u s the tubules open into the posterior end of the midgut (fig. 2, PI. 18); apparently also in Pleretes (Bordas, 1911). In Pieris they open into the hind-gut. Text-fig. 2 shows diagrammatically the relationships of the attachments of the tubules to the gut in Hepialus (B) and Pieris (A). It will be seen that in Hepialus they are outgrowths from the mid-gut. They are strictly comparable in position with those of the Amphipod Crustacea, Myriapoda, and Orthoptera. Their proven endodermal derivation in the Amphipoda is thus

6 288 H. HENSON a strong indication of their fundamentally endodermal nature in Hepialus. Text-fig. 2, A shows the same regions in Pieris brassicae. Here there is a long common duct lined with a chitinous intima and opening into the colon. Where this joins the tubules proper is a ring of embryonic cells exactly like the posterior interstitial ring of the gut. This ring has been observed in many TEXT-FIG. 2. Diagrammatic representations of the attachments of the Malpighian tubules in Pieris (A) and Hepialus (B). Cd, prdctodaeal part of the common duct in Pieris ; Col, colon; II, ileum; Int, chitinous intima covering all parts of the true proctodaeum; M g, mid-gut; Pir, posterior interstitial ring; T ir, interstitial ring of the Malpighian tubules; Tub, endodermal part of the Malpighian tubules. Lepidoptera and is usually referred to as the imaginal ring of the tubules (Ito, 1921). The chitinous intima does not extend beyond this ring. The tubules proper can scarcely be endodermal in Hepialus and ectodermal in Pieris, and the fact that the posterior interstitial ring of the gut marks the end of proctodaeal structures, suggests that so also does the interstitial ring of the tubules. If the view be accepted that the posterior interstitial ring of the gut is the homologue of the lips of the blastopore, we can only explain the presence of the interstitial ring in the tubules as due to their separation from the original blastopore lip during development. This implies that

7 DEVELOPMENT OF PIERIS 289 the common duct in Pier is is a new development and is not represented in Hepialus. The view that the posterior interstitial ring of the alimentary canal is homologous with the lips of the anal half of the blastopore in Peripatus thus enables us to correlate the structure of the Malpighian tubules of insects and explain why they open into the mid-gut in some forms and into the hind-gut in others. It also enables us to regard the tubules of Insects, Crustacea, and Arachnids as homologous and indicates that the association of the tubules with the proctodaeum in insect embryos is a secondary one. Differences of interpretation with regard to the origin of endoderm in insects are notoriously apparent amongst workers in insect embryology (vide Eastham, 1930). Some derive the endoderm from definite anterior and posterior endoderm rudiments; others deny the endodermal nature of the mid-gut because it arises from the blind end of the stomodaeum and proctodaeum. The views advanced in the present paper enable these varying interpretations to be reconciled within a common explanation. 4. DEVELOPMENT OF THE ALIMENTARY CANAL IN PIERIS. (1) Cleavage and Gastrulation. As regards the early development of Pieris, Eastham (1927) has already published a full and well-illustrated account. Accordingly only a condensed history of these phases will be given here, coupled with a new mode of interpretation. For verification of the facts of development reference should be made to Eastham's paper (1927). The unsegmented egg is a large single cell, in the meshes of whose cytoplasm are embedded considerable quantities of yolk. The cleavage nucleus repeatedly divides; some of the daughter nuclei move outwards to the periphery of the egg and form a blastoderm, others remain behind as yolk nuclei. The latter should, I believe, be interpreted as extra-embryonic endoderm. At the poles of the egg and along its dorsal side the blastoderm becomes very thin and is known as the serosa. The rest of the blastoderm thickens and becomes the germ-band. Double

8 290 H. HENSON folds appear at the edge of this germ-band and grow to form a complete amnion over it. In this process the serosa is extended and the germ-band pushed into the yolk. The development up to this point presents some analogies with early mammalian development. After cleavage comes the formation of embryonic membranes which surround a germ-band containing within itself the rudiments of the embryo. Amnion and serosa are readily recognized as extra-embryonic ectoderm. The anterior end of the germ-band produces by proliferation a heap of cells on its upper side. A wave of cell proliferation then passes down the middle line until it reaches the posterior end when another heap is produced similar to that at the anterior end (Text-fig. 3, A). These two heaps of cells were called by Eastham (1927) anterior and posterior mesenteron rudiments. The cells produced by proliferation along the middle line degenerate; they have been recognized as evanescent endoderm both by Eastham (1927) in Pieris and by Mansour (1927) in Calandra. As Eastham's later work (1980) shows, however, much of the anterior mesenteron rudiment is mesoderm. My own observations on Pieris brassicae show that the same is true of the posterior mesenteron rudiment. With Eastham (1930) I therefore prefer to term them anterior and posterior mesendoderm rudiments. The middle region of the germ-band, i.e. the part which produced the evanescent median endoderm, now sinks inwards and is overgrown by the regions lateral to it (Text-fig. 3, B). This middle plate as it is called now lies on the upper side of the middle of the germ-band as the body mesoderm. The stomodaeum and proctodaeum eventually pass inwards precisely on the site of the anterior and posterior mesendoderm rudiments, thus proving that their position is just that of the embryonic mouth and anus in Peripatus. I suggest that these regions are homologous with the lips of the two halves of the blastopore of Peripatus. The site of the production of the anterior mesendoderm rudiment may be called the oral blastoporic area. Similarly the site of the posterior proliferation is an anal blastoporic area. The proliferation along the middle line which produces evanescent endoderm probably represents the closure of the middle part of the blasto-

9 DEVELOPMENT OF PIERIS '291 pore in Peripatus. Although this endoderm degenerates in P i e r i s and C a 1 a n d r a there is no theoretical reason why it should do so in all insects. Indeed its occasional persistence may explain the results of those authors who derive the mid-gut partly from splanchnic mesoderm (vide Eastham's Review, 1930). It follows from this interpretation that the blind ends of the stomodaeum and proctodaeum are not ectodermal but are TEXT-FIG. 3. Later development of Pieris rapae. (A) proliferation from the germ band to produce the anterior and posterior mesendoderm rudiments and the evanescent median endoderm. B, longitudinal section after the sinking inwards of the middle plate to form the body mesoderm. am, amnion; amr, anterior mesendoderm rudiment; eel, ectoderm: ee, evanescent median endoderm; mpm, middle-plate mesoderm; pmr, posterior mesendoderm rudiment; s, serosa. composed of tissue homologous with the lips of the embryonic mouth and anus of Peripatus (i.e. the blastopore lips). This renders it impossible to accept the theory (with its anomalies) that the insect mid-gut is an ectodermal derivation because it arises from the blind ends of these two intuckings. (2) The fore-gut. The condition of the anterior mesendoderm rudiment at the stage described above is shown in fig. 3, PI. 18. It will be observed that ectoderm, mesoderm, and endoderm run indistinguishably into one another at this point.

10 292 H. HENSON This state of affairs is not unexpected on the site of a proliferating, if virtual, blastopore. In fig. 4, PI. 18, is shown the first sign of the intucking of the stomodaeum. This takes place precisely at the position of the anterior mesendoderm rudiment. The oral blastoporic area (still proliferating cells) is thus carried inwards as the blind end of the stomodaeum. The inward movement is accompanied by a peripheral spreading of the cells of the anterior mesendoderm rudiment and their separation into definite mesoderm and endoderm. The peripheral spreading produces particular mesoderm masses round the base of the stomodaeum. These are arranged into paired pre-oral masses in front, a pair of antennal masses at the sides, and a pair of pre-mandibular masses-, behind (fig. 4, PI. 18). The endodermal parts of the original mesendoderm rudiment are now situated entirely above the pre-mandibular mesoderm and around the blind end of the stomodaeum. Its amount is still being increased by proliferation. In a slightly later stage (fig. 5, PI. 18) the stomodaeum is seen to have passed still farther inwards and to have carried the endoderm with it. The line of demarcation between endoderm and ectoderm is not precise and never becomes so. The endoderm now has the form of two masses of cells placed ventrolaterally just posterior to the end of the stomodaeum and connected by a strand across the ventral border of its blind end. The pre-oral mesoderm has separated into two pairs of cellgroups, labral mesoderm in front and epipharyngeal behind, both placed on the dorsal side of the stomodaeum. Antennal and pre-mandibular mesoderm are much as before (fig. 5, PI. 18). Stomodaeum. Starting as a simple tube the stomodaeum continues to grow inwards and becomes folded in a most complicated fashion (fig. 6, PI. 18). Its blind end swells out and becomes extremely thin; the thick stalk then pushes into this swollen end to give what is roughly the shape of a mushroom to the whole organ. In Text-fig. 4 is given a stereogram of it at this period. The outer wall is double and folded right back.

11 DEVELOPMENT OF PIERIS 208 The stalk projects into this region in the form of two folds {v x and its fellow of the other side) which remain separated along the mid-ventral line. Dorsally there is a much smaller fold v 2 behind which the dorsal wall of the stalk is deeply grooved (g) forming a repository in which lie nerve-masses. (Compare also Text-fig. 6.) Owing to the great thickness of the dorsal wall of the stalk (Text-fig. 4) its cavity is restricted and U-shaped. At this stage mesoderm is present between the layers of the TEXT-JTO. 4. Stereogram of the stomodaeum at the stage shown in fig. 6, PL 18. The figure is supposed to represent an optical section in the sagittal plane. v 1 and u 2, folds referred to in the text; g, groove. Compare also Text-fig. 6. folds, but must be left behind during the subsequent elongation of the embryonic folds since there is no mesoderm between the folds of the valve in the larva. In P i e r i s and Vanessa this elongation appears to take place by a lengthening of v 1 and its fellow of the other side which carries v 2 backwards and gives the oesophageal valve the form of a split cylinder. Wigglesworth (1930) has described a trifoliate valve in Cheimab a c c h e; this is possibly a more primitive condition and due to the individual lengthening of v v its fellow, and v 2! with retention of the notches between them. During the course of development the thin outer wall of the blind end of the stomodaeum takes on more and more the NO. 298 u

12 294 H. HBNSON character of endoderm. The process may perhaps be described as a kind of continuous revelation of the endodermal nature of this region. First the ventral border becomes indubitably endodermal (fig. 5, PI. 18), then the process extends laterally and dorsally (fig. 6, PL 18) until finally the whole of this outer wall has the character of endoderm (Text-fig. 5). In this stage TEXT-ITO. 5. Median sagittal section showing the late embryonic condition of the stomodaeum. Camera lueida. cr, crop; end, endoderm; int, interstitial ring; v, valve. it remains crossing the gut cavity as a very thin strand which only breaks down just before hatching. We may regard the process as a dwindling of the oral blastoporic area which thereby becomes reduced from a plate covering the end of the stomodaeum to a circle which persists as the anterior interstitial ring. (See my previous paper (1931), fig. 6, PI. 14.)

13 DEVELOPMENT OF PIERIS 295 Mesoderm and Musculature. The labral mesoderm situated dorsally on the anterior part of the stomodaeum soon passes forwards into the lengthening labrum and backwards, on either side of the frontal ganglion, to the level of the supraoesophageal commissure (Text-fig. 6). It differentiates into the whole of the dorsal system of pharyngeal dilator muscles (except possibly the third posterior dorsal). The posterior dorsal dilator muscles lie on either side of the frontal ganglion and in front of the supra-oesophageal commissure. The epipharyngeal mesoderm rapidly surrounds the posterior end of the stomodaeum (Text-fig. 6), and penetrates between the folds (v v v 2, Text-figs. 4 and 6). It also passes underneath the recurrent nerve and forms a dorsal strip along the main stem of the stomodaeum, as far forwards as the frontal ganglion. Later it passes round on to the lateral and ventral sides in this region also, and differentiates into the transverse and longitudinal muscles of the whole fore-gut. On the pharynx and oesophagus it becomes arranged in six bands (dorsal, dorsolateral, ventro-lateral, and ventral), but on the crop rudiment it remains a continuous sheath. The antennal mesoderm is at first lateral to the stomodaeum. Posteriorly it passes upwards on either side of the recurrent nerve and then above it (Text-fig. 6). This dorsally placed antennal mesoderm eventually surrounds the recurrent nerve, passing between it and the epipharyngeal mesoderm. The cephalic aorta is thus formed almost exactly as described by Eastham (1980). A small portion of the anterior more ventrally placed antennal mesoderm passes inwards on the end of the antennal apodeme (i.e. anterior arm of the tentorium) and produces the middle and posterior ventral dilator muscles of the pharynx. The premandibular mesoderm produces only the sub-oesophageal body. The anterior ventral pharyngeal dilator muscles seem to be much later in appearing, and are probably derived from labial mesoderm. Most likely they are best regarded as labial muscles and not true pharyngeal dilators. (3) The hind-gut. The earliest condition which need be considered is that illustrated in fig. 7, PI. 18. The anal U2

14 296 H. HENSON rn fir TEXT-FIG. 6. Diagrams of serial sections of the stomodaeum to show the distribution of mesoderm. Br, brain; end, endoderm; sob, sub-oesophageal body or premandibular mesoderm; I m, labral mesoderm; r n, recurrent nerve; ant m, antennal mesoderm; ep m, epipharyngeal mesoderm; v lt fold. (Compare also Text-fig. 4, and fig. 6, PL 18.)

15 DEVELOPMENT OF PIERIS 297 blastoporic area has produced a few cells by proliferation and these are not yet separated into mesoderm and endoderm. Before very much proliferation has occurred the proctodaeum begins to pass inwards; separation of the mesoderm then follows. Pig. 8, PI. 18, shows this stage and strongly suggests that the eleventh abdominal segment is left entirely devoid of mesoderm. We are thus led to conclude that the mesoderm surrounding the proctodaeum really belongs to this eleventh Concurrently with its inception the proctodaeum produces on its blind end a pair of lateral bulges from each of which grow out three Malpighian tubules. These may thus be said to arise as a pair of tri-digitate lobes. The three tubule rudiments lie dorso-laterally, laterally, and ventro-laterally on each side (Text-fig. 7). They pass backwards immediately outside the proctodaeal mesoderm to become closely applied to the ectoderm of abdominal segment eleven (fig. 10, PI. 18). The endoderm cells rapidly proliferate and form two compact masses, one on either side in the ninth abdominal segment a little below and behind the blind end of the proctodaeum. These masses may be seen in fig. 9, PL 18. In front they are confluent with the lateral bulges bearing the Malpighian tubules and are connected by a strand across the ventral border of the middle region of the proctodaeum (fig. 8, PI. 18, Text-fig. 8, A). Later they separate from the lateral bulges but always remain attached to the central part (fig. 10, PL 18). To explain the structure of the Malpighian tubules in the larva it has been postulated that they are really endodermal and carry away with them'in their subsequent development a small portion of the blastopore lip. It is impossible to point to particular cells in the embryo and say they mark the blastopore lip. In Text-fig. 7 its position is indicated on purely theoretical grounds which the reader may or may not accept. Its greatest justification is the fact that it enables us to make a consistent comparison between the tubules of Hepialus which open into the mid-gut and those of P i e r i s which open into the hind-gut. Text-fig. 7, A, shows an early stage in which the end of the

16 298 H. HBNSON proctodaeum and the Malpighian tubules are regarded as endodermal and the boundary of the anal blastoporic area is shown in black. We may call this the Hepialus phase of development because of its similarity to that form (cf. Text-fig. 2, B). In the next phase (Text-fig. 7, B) growth of the ventral and dorsal TEXT-FIG. 7. Theoretical reconstruction of the position and development of the boundary of the anal blastoporic area. El a, boundary of anal blastoporic area (black); Cd, proctodaeal part of the common duct of the tubules which later increases in length; hit, interstitial ring of the gut (main part of the blastopore boundary); Int t, interstitial or imaginal ring of the Malpighian tubules (i.e. cut-off part of blastopore boundary); M g, mid-gut cells recognizable from the first; Pr, proctodaeum. sides of the proctodaeum has carried the boundary of the blastoporic area forwards above and below, but has left it looping back under the stalks of the tubules at the sides. The reality

17 DEVELOPMENT OF PIEEIS 299 of this process may be indicated by reference to fig. 9, PI. 18, where it has carried the mesoderm forwards as a dorsal and ventral tongue between the tubule attachments. By a continuance of this forward growth, a portion of the boundary of the blastoporic area is left behind on the tubule stalks (Text-fig. 7, C, D), where it later reveals itself as the interstitial ring of the tubules. The main part of the boundary goes on to become the posterior interstitial ring of the gut. Eegions of hind-gut. The relationships of the proetodaeum to the posterior end of the body in the stage shown in fig. 8, PI. 18, are better revealed in Text-fig. 8, A, which is a diagram of a coronal section through the proctodaeum. This phase is rapidly followed by one in which the eleventh abdominal segment becomes intucked to form the rudiment of the anterior rectum (Text-fig. 8, B; fig. 10, PI. 18). The blind ends of the Malpighian tubules come into very close association with this intucked segment and eventually develop complicated anatomical relations with it. Meanwhile the proctodaeum proper has increased in length as already described. Although the attachment of the tubules would appear at first sight to divide it into colon and ileum, the mesoderm attachments clearly show that this is not so. The mesoderm naturally divides the proctodaeum into ileum and colon (Text-fig. 9), but the tubule attachments are concerned entirely with the colon. During the lengthening of the proctodaeum the posterior half of the tenth abdominal segment also becomes intucked and forms the rudiment of the posterior rectum (Text-fig. 8, C, and Textfig. 9). The mesoderm of the eleventh abdominal segment is, as we have seen, early passed on to the proctodaeum; thus it comes about that the anterior rectum is always devoid of muscles or other mesoderm elements. The mesoderm continuously grows forwards as the proctodaeum lengthens, and slides like a slip ring over much of the common ducts of the tubules. From the posterior end of the true proctodaeum to a point somewhat in front of the opening of the tubules into the gut, the mesoderm arranges itself in six

18 300 H. HENSON longitudinal bands (dorsal, dorso-lateral, ventro-lateral, and ventral). On the anterior end the mesoderm arranges itself in TEXT-FIG. 8. Diagrams to illustrate the development of the hind-gut. 9, 10, 11, abdominal segments; a r, anterior rectum; col, colon; il, ileum; m, mesoderm; pr, posterior rectum; t, tubule; tp, proctodaeal common duct of tubules; end, endodermal mid-gut. a continuous sheath and not in bands. The anterior region is ileum, the posterior colon (Text-fig. 9, E and C). The common ducts of the tubules run underneath the ventrolateral muscle-bands of the anterior third of the colon and have

19 DEVELOPMENT OF PIERIS 301 no muscles of their own. Where they become free from the gut they take this ventro-lateral band with them and thus produce a short piece of the colon with only four muscle-bands (Textfig. 9, D). The anterior and posterior sphincter regions are really only the anterior and posterior ends of the colon especially heavily provided with muscles. In the final stage the blind end of the proctodaeum reveals itself as endodermal (Text-fig. 9, A) by developing an endoderm lamella across the gut cavity just as in the case of the stomodaeum. Mid-gut. Little need be said with regard to this since, to Eastham's account, I have only to add that the blind ends of both stomodaeum and proctodaeum are also endodermal, although transitional and destroyed just before hatching. 5. SUMMARY. 1. The interstitial (imaginal) rings of the insect gut are interpreted as homologous with the lips of the embryonic mouth and anus of Peripatus (i.e. the blastopore lips). 2. The Malpighian tubules of Amphipod Crustacea, L i t h o - bius, Stenopelmatus (Orthoptera), Hepialus (Lepidoptera), Calliphora (Diptera) are all appendages of the posterior end of the mid-gut and endodermal. 3. The Malpighian tubules of Pieris, although hind-gut appendages must be homologous with those of Hepialus. They are composed of three regions, (1) the functional parts of endodermal derivation, (2) the interstitial or imaginal ring which is probably derived from the posterior interstitial ring of the gut, (3) the common duct of proctodaeal origin. 4. The germ-band of the Lepidopterous embryo has a closed blastopore or primitive streak composed of two circular areas, anal and oral, connected by a median strand. The anal and oral blastoporic areas produce the anterior and posterior mesendoderm rudiments. 5. The development of the stomodaeum and proctodaeum shows that the following characteristics may be ascribed to the various parts of the gut.

20 302 H. HENSON TEXT-FIG. 9.' Longitudinal (coronal) and transverse sections of the hind-gut. Arrows point to the regions from which the transverse sections are taken. B, anterior rectum and the membranes covering the terminal parts of the tubules. C, Colon. D, Anterior end of colon with four muscle bands. E, ileum. References as in Text-fig. 8.

21 DEVELOPMENT OF PIERIS 303 Pharynx. The oral or 'proximal' end of the stomodaeum. Its dorsal dilator muscles are derived from labral mesoderm, its ventral dilators from antennal mesoderm, and its circular and longitudinal muscles from epipharyngeal mesoderm arranged in six bands. Oesophagus. The longitudinal and circular muscles are derived from epipharyngeal mesoderm arranged in six bands. Crop. Longitudinal and circular muscles derived from epipharyngeal mesoderm not arranged in bands but as a continuous encircling sheath. Oesophageal Valve. Primarily trifoliate and formed by outgrowth of three embryonic folds; devoid of mesoderm. Anterior Interstitial Eing. The persistent part of the oral blastoporic area and homologous with the lips of the embryonic mouth of Peripatus. Mid-gut. Bndodermal, and formed from those parts of the blastoporic areas internal to the blastopore lips. Posterior Interstitial Eing. The persistent part of the anal blastoporic area and homologous with the lips of the embryonic anus of Peripatus. 11 e u m. The anterior end of the proctodaeum where the mesoderm forms a complete sheath of muscles not arranged in six bands. Colon. The mesoderm is arranged in six bands. The sphincter regions are really the anterior and posterior ends of the colon. The common ducts of the Malpighian tubules enter the colon and derive their muscle-sheath from its ventrolateral bands. Anterior Eectum. The intucked eleventh abdominal segment; closely associated with the terminations of the Malpighian tubules. Its mesoderm early passes on to the proctodaeum so that in subsequent stages it is devoid of mesodermal structures. Posterior Eectum. The intucked posterior half of the tenth abdominal segment. Musculature derived from the tenth abdominal somites.

22 304 H. HENSON 6. BIBLIOGRAPHY. Baldwin Spencer (1885). "The Urinary Organs of the Amphipoda", 'Quart. Journ. Micr. Sci.', 25. Bordas (1911). "L'Appareil digestif et les tubes de Malpighi des larves des Lepidopteres", 'Ann. Sci. Nat. Zool.', ser. ix, torn. xiv. Davis (1927). "The Anatomy and Histology of Stenopelmatus", 'Univ. Calif. Pub. Ent.' Eastham (1927). "A Contribution to the Embryology of Pieris rapae", 'Quart. Journ. Micr. Sci.', 71. (1930). "The Embryology of Pieris rapae Organogeny", 'Phil. Trans. Roy. Soc.\ B., 219. (1930). "The Formation of the Germ Layers in Insects", 'Biol. Reviews', 5. (This paper contains a very full bibliography.) Henson (1931). "The Structure and Post-Embryonic Development of Vanessa urticae. I. The Larval Alimentary Canal",' Quart. Journ. Micr. Sci.', 74. Ito (1921). "On the Metamorphosis of the Malpighian Tubes of Bombyx mori", 'Journ. Morph.', 35. Korschelt and Heider. 'Text-book of Embryology', Pt. II. Leuzinger, Wiesmann, und Lehmann (1926). ' Zur Kenntniss der Anatomie und Entwicklungsgeschichte der Stabheuschrecke (Carausius morosus)', G. Fischer, Jena. Mansour (1927). "The Development of the Larval and Adult Mid-gut of Calandra oryzae, the Rice Weevil", 'Quart. Journ. Micr. Sci.', 71. Perez (1910). "Recherches histologiques sur les metamorphoses des Muscides", 'Arch. Zool. Exp.', 5 me ser. Sedgwick (1885). "The Development of Peripatus capensis", 'Quart. Journ. Micr. Sci.', 25. Wigglesworth (1930). "The Form of the Peritrophic Membrane in Insects with special reference to the Larvae of Mosquitoes", ibid., 73. EXPLANATION OF PLATE 18. LETTERING. AM, amnion; AMP, ampulla of Malpighian tubule; AMR, anterior mesendoderm rudiment; ANT M, antennal mesoderm; 10th A 8, 11th AS, abdominal segments 10 and 11; EOT, ectoderm; END, endoderm; EP M, epipharyngeal mesoderm; F G, frontal ganglion; H O, hind-gut; INT, intima; LM, labral mesoderm; LT, lateral lobe of proctodaeum giving rise to the Malpighian tubules; MES, mesoderm; M O, mid-gut; PO, protocerebral ganglion; PIR, posterior interstitial ring; PMM, premandibular mesoderm; P M R, posterior mesendoderm rudiment; POM, pre-oral mesoderm; PR, proctodaeum; RN, recurrent nerve; SOO, sub-oesophageal ganglion; ST, stomodaeum; T, Malpighian tubule;

23 DEVELOPMENT OF PIERIS 305 T 0, tritocerebral ganglion; T T, terminal part of Malpighian tubule; YK, yolk. All figures drawn with Camera lucida. Fig. 1. Longitudinal section through the attachment of the Malpighian tubule in Lithobius. (Compiled from four sections in the same series.) Fig. 2. Same in Hepialus humuli (half-grown larva). Fig. 3. Longitudinal sagittal section of the front end of a 24-hour embryo of Pieris brassicae. The anterior mesendoderm rudiment is shown as continuous with the ectoderm on the one hand and the body mesoderm on the other. Fig. 4. Same in a 30-hour embryo. The stomodaeum is just beginning to appear. Fig. 5. Same in a 48-hour embryo. Fig. 6. Longitudinal sagittal section through the stomodaeum in a 72-hour embryo (Pieris brassieae). Fig. 7. Longitudinal sagittal section through the posterior end of a 30-hour embryo. The mesendoderm rudiment is shown immediately above the site of appearance of the proctodaeum. Fig. 8. Same in a 48-hour embryo. The proctodaeum has now passed inwards and endoderm and mesoderm are completely separated. Fig. 9. An obliquely transverse section across the inner end of the same stage as fig. 8. On the left of the figure the origin of the Malpighian tubules from a lateral lobe is shown. On the right the three tubules are seen passing backwards. The endoderm is also seen to be in the form of a pair of lateral masses connected by a thin median plate or strand. Fig. 10. A coronal longitudinal section of the proctodaeum in a 30-hour embryo.