Paper - A monstrous twin embryo in a lizard tiliqua scincoides (1932)
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Willis RA. A monstrous twin embryo in a lizard, tiliqua scincoides . (1932) J Anat. 66: 189-201. PMID 17104367
|lizard (Tiliqua scincoides) with twinning.
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A Monstrous Twin Embryo in a Lizard, Tiliqua scincoides
By Rupert A. Willis, M.D. Pathologist to the Alfred Hospital, Melbourne, Victoria, Australia
From the Baker Research Institute, Alfred Hospital, Melbourne
When dissecting some Australian Blue-tongued Lizards (Tiliqua scincoides), a pregnant female was encountered bearing eight ova in an early stage of development. Each oviduct contained four eggs of normal size, i.e. approximately 2-2 cm. in diameter, and each ovary contained four prominent haematoma-like areas each about 6mm. in diameter, which proved microscopically to be developing corpora lutea.
Fig. 1. A normal embryo seen through a hand lens. x 8.
Each of the eight ova still consisted largely of yolk, but presented at one pole a transparent amniotic cavity 8mm. in diameter. On opening the amniotic sacs, seven of the contained embryos were normal, but the eighth ovum contained a twin monster.
The appearance of the normal embryos is indicated in fig. 1. The uncoiled cranio-caudal length, measured from the roof of the hind-brain to the convexity of the back just proximal to the lower limb buds, was 5-5 mm. The maximum antero-posterior diameter, measured from the fore-brain to the dorsum just cranial to the level of the upper limb buds, was 4-5 mm. The maximum transverse width, from cornea to cornea, was 2mm. The following features were visible through a hand lens:
(a) Darkly pigmented eyes, each 1 mm. in diameter, with a pupil 0-3 mm. in diameter, and a distinct choroidal fissure extending dorso-caudally from the pupil.
(b) Fore- and mid-brain, faintly demarcated from each other by a shallow depression.
(c) Prominent spherical corpora bigemina 1 mm. in diameter.
(d) A thin-roofed hind-brain with transparent walls enabling one to see clearly the contour of the floor of the ventricle.
(e) Faintly visible otic vesicles on either side of the hind-brain, each 0-5 mm. in diameter.
(f) A prominent heart, with distinct dark auricle and pale ventricle, and three branchial arch vessels visible, extending from the heart region dorsally to the dorsal aorta.
(g) Vitelline vessels emergent in the body stalk and communicating with a dark vascular hepatic anlage just caudal to the heart.
(h) Distinct somites extending from the hind-brain region to the coiled tail.
(i) Upper limb buds of two segments arising opposite the ventricle of the heart and projecting laterally and caudally from the body.
(j) Lower limb buds, doubtfully segmented, arising at the root of the coiled tail, and projecting laterally and ventrally from the body.
Fig. 2, The malformed twin embryo seen through a hand lens. x &
The seven normal embryos were embedded in paraffin and serially sectioned, some frontally, some sagittally and some transversely. A study of these series of sections confirmed the normal structure of the seven embryos, and, though these specimens were of value in identifying parts of the abnormal embryo, no further description of them is here necessary.
The monstrous twin, viewed with a hand lens, is depicted in fig. 2. It consisted apparently of two embryos, A and B, imperfectly developed and partially fused back to back. Its cranio-caudal length was 6 mm. The cephalic region presented four hemispherical projections each bearing a pigmented area evidently representing an imperfectly developed eye. No distinct pupils or choroidal fissures could be discerned, and the pigmentation of each eye was less well defined than in the normal embryos and did not form a complete circle. No indications of the various regions of the brain, as seen in the normal embryos, could be discovered. A clear vesicular projection, H, was present on one aspect of the monster in the cardiac region. Situated one on each side of this were two smaller evidently blood-containing projections joined by a narrow bridge cranial to the vesicle, L. Caudally the monster consisted of two diverging coiled tails. It was observed with some surprise that neither limb buds nor somites could be discovered macroscopically. No well-defined body stalk could be seen, but some delicate membranous strands of tissue clung around the caudal parts of the monster.
Serial paraffin sections 10u thick and numbering 600 were prepared and stained by haemalum and eosin. The following descriptions are based on a study of these sections and of partial reconstructions made from them.
II. Description of the Double Monster
The accompanying photographs and diagrams explain more lucidly than words the remarkable structure of the malformed twin embryo, a general summary of which may, however, be made under the following heads.
(1) The entodermal epithelia
In the sections of the upper half of the monster (figs. 3-5) it is at once apparent that this is clothed externally not by ectoderm but by alimentary entoderm. This is evident, not only from the character of the epithelium itself, but by its relationship to the notochords and to the hepatic rudiments, and by the absence of any entodermal derivatives centrally situated within the double embryo.
In the cranial region the otherwise complete entodermal covering is interrupted by the presence of open gill clefts, three pairs in one twin and two pairs in the other (figs. 8 and 4, G). The four highest or first clefts are widely dilated by the partial protrusion through them of the eyes, a remarkable relationship to be discussed later. Another deficiency in the cephalic entoderm of each twin (not seen in the figures) is a slit-like communication with Rathke’s pouch. This aperture is of course due to the absence of the primitive bucco-pharyngeal membrane which otherwise would still separate the entodermal covering from the stomodeal region. It may be noted here that the bucco-pharyngeal membrane had completely disappeared in all the normal embryos. Clearly in the cranial part of the monster the investing entoderm must have been subjected to great mechanical distension by the growth of the centrally situated brain, eyes and other structures, and the resulting attenuation of the layer of alimentary epithelium is evident in figs. 3-5.
Sections in the cardio-hepatic region present a less complete entodermal investment. While the whole of one aspect of the twin body (the reverse of fig. 2) is completely clothed by entoderm, mesodermal structures, namely the serosal covering of the liver and heart of twin A, appear externally on the aspect presented in fig. 2. From figs. 5 and 6 the visible projections in fig. 2 may clearly be identified as the liver and heart of twin A protruding at the upper angle of deficiency of the entodermal covering. At the margins of this deficiency the entoderm does not cease, but is reflected off the body of the monster along with a thin mesothelial layer continuous with the coelomic lining. This reflected splanchno-pleuric lamina forms a delicate membrane which was torn in removing the embryo from its amniotic sac (XX in fig. 5-9). Clearly below the level of reflection of this membrane mesodermal structures clothed by coelomic endothelium must everywhere project on the surface of the monster.
Figs. 3 to 9. Diagrams of cross-sections of the double monster, indicating the identity of the various parts. Ectodermal structures are blue, entodermal structures black, and mesodermal structures stippled. A and B denote the two members so specified in fig. 2, and the following symbols are used throughout: C, coelom; D, fused diencephalic rudiments (compare figs. 10 and 11); H, eye; F, olfactory pit; G, gill cleft; H, heart; J, lung bud; K, kidney; L, liver; N, notochord; O, otocyst; P, pancreas; Q, central amniotic space; R, hind-brain; S, spinal cord; 7, upper limb bud; W, tubular hind-gut; X, torn margin of splanchnopleuric membrane. In cranio-caudal order, the following sections are illustrated: fig. 3, section 140; fig. 4, section 180; fig. 5, section 220; fig. 6, section 240; fig. 7, section 300; fig. 8, section 360; fig. 9, section 400. Magnification of all sections, 25 diameters.
The caudal half of each twin contains a closed tubular prolongation of entoderm representing the hind-gut, the continuity of which with the external entodermal layer of twin A is seen in figs. 8 and 9. The caudal extremity of each hind-gut gives off an allantoic diverticulum of which only the torn pedicle is left, the remainder of this membrane having been lost in removal of the embryo from the egg.
The distribution of the respiratory rudiments is peculiar. Just as the foregut entoderm of each embryo has necessarily been unable to form a closed alimentary tube, so also there has been no formation of the median tubular respiratory diverticulum, and in each twin there are two widely separated caudally directed short lung buds arising from the external entodermal covering at points representing the true ventral wall of the unclosed fore-gut. Two of these are seen at J in figs. 5 and 6.
The liver rudiments and their relations to the entodermal envelope and to the vascular systems are seen in figs. 5-7. The liver of member B lies entirely within the “body” of the dual embryo, which on this aspect (the reverse of fig. 2) is clothed by a continuous external sheath of entoderm. On the aspect viewed in fig. 2, however, we have seen that this sheath is incomplete at the hepatic level, so that the liver of member 4 with its coelomic covering is enabled to protrude externally. Associated with this eversion of the liver at the level of reflection of the attenuated splanchno-pleura is the herniation of the heart from the widely open coelomic cavity at a level caudal to the liver itself (compare fig. 2 with figs. 5-7).
Each of the pancreatic rudiments P is situated normally in the dorsal mesentery (figs. 7 and 8).
(2) Ectodermal epithelia
As is evident in figs. 4-9, the cutaneous ectoderm lines a central cavity Q which is really an amniotic gulf extending cranially from between the divergent tails of the monster. The backs of the two-member embryos lie opposite one another with this amniotic gulf between, and at the lateral margins of each member the somatopleure turns dorsally to meet and become continuous with that of the other member. This purely lateral fusion of the dorsally opposed embryos with the resulting enclosure of part of the amniotic space extends nearly to the most cranial part of the monster, where, however, total fusion of the two heads is present and the amniotic gulf comes to a blind end. The topography of this region has become very distorted and complex as the result of growth of the brains and other cephalic structures. The enlarging fused head, being limited cranially by the externally investing layer of entoderm, has expanded caudally into and distended the upper blind end of the space between the two embryos. Thus the most cranial limit of this space is a circular sulcus or fornix formed where the trunk entoderm is reflected caudally on to the central caudally projecting cephalic mass. As a result of the expansion of this mass the gill clefts have become dilated, and, as already noted, the four eyes of the enlarging heads partially protrude through the four first clefts. We can now appreciate the reason for the early macroscopic observation that the eyes of the monster, unlike those of the normal embryos, lacked detail and were indefinite in outline. This was because the eyes were not truly superficial as under normal conditions, but were partially obscured by the surrounding lips of the gill clefts which formed, as it were, their embrasures. It is as if fused twins with a common gut and a large solid head were-to be turned inside out and were then to peer out on the world through their own distended gill clefts. Indeed this simile, though only a simile, is very helpful in enabling one to visualise the general structure of the malformation under discussion.
All structures normally derived from the cranial cutaneous ectoderm are represented in the fused head region of the monster. Olfactory pits F’, optic lenses, Rathke’s pouches, and otic vesicles O are present (fig. 3). The distorted Rathke’s pouches deserve special comment. Each commences in the cranioectodermal fornix already described, and extends upwards between the entoderm externally and the fused brain internally. As already remarked, there is a slit-like communication between each pouch and the adjacent entoderm, indicative of the disappearance of the bucco-pharyngeal membrane. The fundal extremities of the two pouches extend almost to the vertex of the monster, where they lie close together, but not in contact, against the upturned floor of the fused brain.
Four pairs of limb buds are present, the upper ones, 7’, appearing as rounded projections at the sides of the dorsa of the embryos about the centre of the trunk region (figs. 7-9), while the lower pairs are situated as in the normal embryos in the coiled caudal region.
(3) Neural ectoderm
The central nervous system consists of a fused brain and two separate brain stems and spinal cords. The latter are normal, but the cerebral region requires special description. Figs. 10 and 11, made from plaster of Paris reconstructions , explain the peculiar topography of the parts. Clearly the most cranial part, H, of the cerebral rudiment is really the floor or ventral aspect of the fused brain, for it is on this aspect that the entoderm and the two Rathke’s pouches lie. Conversely the most caudal or dependent part of the central cephalic mass, P, is the true cerebral vertex. In this region some irregularities in the walls of the neural tube (seen in fig. 4) probably represent the pineal rudiments. The brains are thus inverted in position.
Fusion of the two cerebral rudiments has occurred just anterior to their diencephalic regions, for the fused central portion, HP, bears the two pairs of eyes, and the position of the two Rathke’s pouches already described indicates that, on the upturned cerebral base H, fusion has been in the hypophyseal region. No separate hypophyses are present, and it is probable that these structures are fused together and are represented in the central cranially projecting diencephalic base H.
In each hind-brain region there is an abnormal acute dorsal flexion, the metencephalon being doubled back on itself and presenting an acute ventral convexity X, projecting cranially and nearly level with the diencephalic base H.
The optic vesicles are of normal structure and are attached to the fused diencephalon by hollow optic stalks situated nearly transversely tangential to the upper (or ventral) aspects of the vesicles.
Fig. 10. Drawing of reconstructed model of the brains viewed from the same aspect as fig. 2. SS, spinal cords; XX, convexities on ventral walls of hind brain produced by acute dorsal flexure of neural tubes; HP, fused diencephalic parts of the two members, H, being the basal or hypophyseal region and P being the vertical or pineal region. Dotted lines NN indicate position of notochords, the cranial ends of which terminate in the acute angles V. Optic vesicles denoted by coarse dotted lines. x 20.
Fig. 11. View of brains at right angles to fig. 10. x 20.
(4) The notochords
In all sections the two notochords lie just beneath the external entoderm and on the ventral or outer aspects of the neural tubes. In the head region the position of the notochords is indicated by NN in fig. 10. Each follows the flexures of the brain stem, extending over the cranial convexity, X, of the hind-brain, and then turning caudally to terminate in the acute angle, V, between the brain stem and the cranially projecting base of the diencephelon H. These relationships serve to verify what has been stated above regarding the identity of the various regions of the distorted brain.
(5) Mesodermal structures
As already noted, where the entoderm ceases to clothe the monster externally and is reflected off the organism in an attenuated splanchopleuric membrane XX, coelomic mesothelium must necessarily appear on the exterior of the more caudal parts of the monster. Thus in figs. 5-9 we see at various levels the projection of the heart, liver, kidneys, genital ridges and other structures from the open coelom. As we have seen, this presentation of mesodermal structures on the surface of the monster is greater on the aspect viewed in fig. 2 than on the reverse side. The normal coiled tails possess a complete ectodermal investment, so that here no mesodermal structures appear externally.
The various viscera and their mesenteries have largely developed normally, though necessarily distorted by the peculiar configuration of the parts. There is inter-communication and partial fusion of parts of the two vascular systems, the details of which, however, are beyond the scope of this paper.
Finally we may note why the mesodermic segments and the limb buds were obscured on macroscopic examination of the monster. It will be clear from figs. 7-9 that the somites and limbs of the embryos were not superficially situated as in normal embryos, but were hidden from external inspection by more external tissues. The true “exterior” from which segmentation and limb buds would be visible is the inter-embryonic amniotic gulf Q.
III. Interpretation of the Malformation
I was for long at a loss for an embryological explanation of the unique structure of this twin monster, but after repeated study of all features of the specimens, I believe the following interpretation to indicate the probable mode of origin.
From the foregoing description it is clear that, while two complete individuals are represented in the monster, the alimentary entoderms of the two members are continuous with one another. It must be assumed, therefore, that the early stage of development of the twin embryonic area was as depicted in fig. 12, I. Two individual ectodermal laminae with contiguous cephalic ends overlay a common entodermal sheet representing the twin entoderms in cephalic continuity with each other. With the growth of the embryos and the enlargement of the amniotic cavity, the egg, which was only of normal size, became too small to accommodate the two members in the position represented in fig. 12, I. Mechanical flexion of the embryos therefore occurred in the hindbrain region, the tails becoming approximated to each other, and the twins thus being folded back to back, as depicted in fig. 12, IJ. In this way was produced the cerebral inversion and the acute dorsal flexures of the hind-brains seen in fig. 10. Further growth of the embryos and absorption of yolk was accompanied by the normal “mushrooming” up of the entire monster into the enlarging amniotic sac. Thus the condition represented in fig. 12, III, essentially identical with the specimen described, was attained. In removing the monster from the egg, the delicate amnio-splanchno-pleuric membrane would be ruptured at XX, its attachment to the body of the monster, and the allantoic diverticula also would be torn in the same region.
Now cross-sections P and Q of the condition represented in fig. 12, II, would appear as in fig. 18, and it will be evident that, if the external attenuated layers be torn away at XX, these hypothetical cross-sections are essentially identical with the actual cross-sections shown in figs. 8 and 9.
Fig. 12. Diagrams of hypothetical mode of formation of the monster. A, amniotic cavity; Y, yolk; XX, points of detachment of amnio-meso-entodermal membrane. Coelomic split not represented in dotted mesoderm, and nervous system not represented as distinct from ectoderm.
Fig. 13. Hypothetical sections P and @Q of fig. 12. A, gulf of amniotic cavity between embryos; Am., amniotic membrane; C, coelom; Ent., entoderm; XX, points of tearing of attenuated splanchnopleure. Mesoderm dotted. A Monstrous Twin Embryo in a Lizard 201
I conclude, therefore, that the malformation described arose by mechanical folding back to back of cephalically fused twins, resulting in (a) inclusion of an interembryonic portion of the amniotic space between the opposed dorsa, (b) complete eversion of the entoderm which thus clothes the monster externally and has been unable to produce a closed alimentary canal except in the caudal regions, (c) dorsal continuity of the somatopleures of the two members. I have been unable to discover in the literature any record of a condition at all resembling that described.
I am indebted to Prof. Wood Jones of the Melbourne University for examining the specimen and confirming my conclusions regarding its mode of formation.
Cite this page: Hill, M.A. (2021, December 4) Embryology Paper - A monstrous twin embryo in a lizard tiliqua scincoides (1932). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_monstrous_twin_embryo_in_a_lizard_tiliqua_scincoides_(1932)
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