The Ultrastructure of the Protonephridium of the Actinotroch Larva (Phoronida)

The actinotroch larva of Phoronis muelleri has a pair of protonephridia located beneath the tentacle ring and draining the blastocoel; each protonephridium is composed of about 25 solenocytes and a nephroduct which opens in a nephropore on the ventral side of the metasome. The neck of the solenocytes consists of bars, mutually interconnected by a fenestration lamina. Inside the neck microvilli originate proximally in the proximal intrachoanal field and extend through the neck into the nephroduct. There is no canal cell. In cross section the nephroduct is composed of 5–7 monociliary cells, with the cilium protruding through a border of microvilli and extending into the nephroduct. The whole protonephridium is surrounded by a basal lamina. Comparisons of the actinotroch protonephridium with those of other groups have not revealed any convincing homologies. The protonephridia of the protostomians are all considered to be of ectodermal origin, while the cyrtopodocytes of Branchiostoma are mesodermal. The protonephridium of the actinotroch is ectodermal.     

The excretory organs in Sphaerodorum flavum (Phyllodocida, Sphaerodoridae): a rare case of co-occurrence of protonephridia, coelom and blood vascular system in Annelida

The excretory organs of Sphaerodorum flavum (Sphaerodoridae) were investigated by TEM and reconstructed from serial ultrathin sections. These organs are segmentally arranged paired protonephridia, which are in close association with a well-developed blood vascular system. Each protonephridium consists of a terminal part made up of two monociliary terminal cells (solenocytes), and a nephridioduct, formed by two cells. The two solenocytes lie close together. Each cilium is surrounded by 12 microvillar rods projecting from the perikaryon of each solenocyte. These rods form a weir-like structure in the coelomic space. The distal part of the weir is embedded in the proximal nephridioduct. The largest part of the cell bodies of the solenocytes, containing the nucleus, is lateral or basal to the weir-like structures. The lumen of the nephridioduct is formed by two multiciliated cells, which enclose the extracellular nephridial canal one behind the other. The canal opens through the nephropore beneath the cuticle without penetrating the cuticle. Both nephridioduct cells are surrounded by a blood vessel, which is partially folded into several layers. The significance of a simultaneous occurrence of protonephridial excretory organs and a well-developed blood vascular system as well as coelomic cavities is discussed. The results of this investigation indicate a close relationship of Sphaerodoridae to Phyllodocidae instead of to Syllidae within the Phyllodocida. Accepted: 27 November 2000     

Ultrastructure of the metanephridia of Terebratulina retusa and Crania anomala (Brachiopoda)

Adult specimens of Terebratulina retusa and Crania anomala have one pair of metanephridia. Each metanephridium is composed of a ciliated nephridial funnel (nephrostome) and an outleading nephridial canal, thus, these organs are open ducts connecting the metacoel of the animal with the outer medium. In both species, the inner side of a nephrostome is lined by a columnar monociliated epithelium which contacts the coelothel within one of the two ileoparietal bands. The coelothel contains basal filaments (in C. anomala these are definite myofilaments). The canal epithelium also consists of monociliated columnar cells which differ from the nephrostome epithelial cells in size and some cell components. Within the nephropore, the canal epithelium makes contact with the so-called inner mantle epithelium which lines the mantle cavity. The nephrostome epithelial cells and the canal epithelial cells never contain any contractile filaments. There are always continuous transitions between these different epithelia and distinct borders cannot be observed. The present results, especially in comparison to Phoronida, do not contradict the hypothesis of a coelothelially derived nephridial funnel and an ectodermal nephridial duct in Brachiopoda. But with regard to the differences between Phoronida and Brachiopoda (larval protonephridia and podocytes in the adults are unknown in Brachiopoda), further investigations have to be done to test the hypothesis of such heterogeneously assembled metanephridia.     

Ultrastructure and development of the nephridia in Anaitides mucosa (Annelida,Polychaeta)

Different developmental stages (trochophores, nectochaetae, non-mature and mature adults) of Anaitides mucosa were investigated ultrastructurally. A. mucosa has protonephridia throughout its life; during maturity a ciliated funnel is attached to these organs. The protonephridial duct cells are multiciliated, while the terminal cells are monociliated. The single cilium is surrounded by 14 microvilli which extend into the duct lumen without coming into any contact with the duct cells. Corresponding ultrastructure and development indicate that larval and adult protonephridia are identical in A. mucosa. Differences between various developmental stages can be observed only in the number of cells per protonephridium. A comparison between the funnel cells, the cells of the coelothel and the duct cells reveals that the ciliated funnel is a derivative of the duct. Due to the identical nature of the larval and postlarval protonephridia, such a funnel cannot be a secondary structure. In comparison with the mesodermally derived metanephridial funnel in phoronids it seems likely that the metanephridia of annelids and phoronids evolved convergently.     

The ultrastructure of the protonephridial system of Götte's larva of Stylochus mediterraneus (Polycladida, Platyhelminthes)

The protonephridial system of Götte's larva of Stylochus mediterraneus was studied by electron microscopy. There is one protonephridium on each side of the body, formed by one terminal and one canal cell. The terminal filtration apparatus is formed by a single cell (the terminal cell) with several globular processes, the largest of which includes the nucleus. Fingers of cytoplasm (leptotriches) from each process penetrate the lumen surrounding the bundle of cilia and fingers from adjacent processes interdigitate to form a pattern of convoluted slits which constitute the weir. The single canal cell is inserted internally to the terminal cell at the top of the weir and encloses the lumen without a junction. Septate junctions are present between the terminal and canal cells. The lumen of the canal cell is smooth-walled for most of its length and cilia arise and terminate at all levels of the terminal and canal cells. Posterior to the larval mouth opening, the canal cell crosses the epithelium and the lumen ramifies to form the excretory opening. The terminal apparatus closely resembles that found in the freshwater planarian Bdellocephala brunnea .     

扩张莫尼茨绦虫(绦虫纲:圆叶目)的原肾管及原肾管概念的评述

本研究应用透射电子显微镜研究了扩张莫尼茨绦虫原肾管的细胞学特征 ,莫尼茨绦虫原肾管的焰茎球为一个过滤器结构 ,类似于“挡河坝”样构造 ,此构造由端细胞和近管细胞外突形成的肋条 (或称杆 )相互交错排列而成。肋条之间由细胞外物质构成的“膜”结构连接 ,过滤作用通过该“膜”发生。焰细胞与近管细胞交界处有裂缝或孔与细胞外的结缔组织 (实质组织 )相通 ;原肾管的毛细排泄管细胞质索之间没有隔状联结 ;毛细排泄管及排泄管的管腔内有大量珠状微绒毛突起以增加表面积。从扩张莫尼茨绦虫及其它一些无脊椎动物原肾管的研究结果表明 ,原原肾管概念将焰细胞作为封闭的盲端已不再合适 ,需要进行修订 ,建议修订为 :原肾管是一种焰细胞系统 ,通常由焰细胞、管细胞和肾孔细胞组成 ,焰茎球作为过滤装置与周围的结缔组织 (实质组织 )有或没有裂缝 (孔 )相通     

THE FUNCTIONAL ORGANIZATION OF FILTRATION NEPHRIDIA

(1) Based on the classical studies of Goodrich, protonephridia are believed to be phylogenetic antecedents of metanephridia. It is argued here that the primary factor determining the type of nephridium expressed is body size rather than phylogenetic status. (2) The proposed model defines a nephridium functionally and predicts two general configurations for filtration nephridia in animals. (3) Application of the model to metanephridial and protonephridial systems indicates differences in the sites of ultrafiltration and mechanisms of pressure generation. (4) Metanephridial systems function by muscle-mediated filtration of vascular fluid into a coelomic space before modification by an excretory duct. (5) Protonephridial systems function by cilia-mediated filtration of extracellular fluid into the lumen of a protonephridial terminal cell before modification in an adjoining duct. (6) The model predicts a correlation between animals with blood vessels and metanephridia, and animals without blood vessels and protonephridia. The correlation is shown to be nearly perfect. (7) Exceptions to the model are discussed. (8) Original experimental evidence is given for the permeability of the protonephridial terminal cell to iron dextran and its reabsorption by the protonephridial duct in the polychaete, Glycera dibranchiata. (9) Experimental data for proto- and metanephridial systems are summarized and shown to support the proposed model. (10) The ultrastructure of the exceptional amphioxus ‘protonephridium’ is reviewed and original data are presented. Its organization is structurally and perhaps functionally intermediate between proto- and metanephridial systems. (11) An original ultrastructural comparison is made of monociliated nitration cells in a size range of larval invertebrates from five phyla. Filtration cells that are structurally intermediate between protonephridial solenocytes and metanephridial podocytes are noted in larvae intermediate in body size between the two extremes. The comparative data suggest that (i) podocytes and solenocytes are homologous cells and (ii) that body size is correlated with which of the two designs is expressed. (12) The fates of larval podocytes are followed through metamorphosis in three species. The results confirm the equivalence of podocytes and solenocytes as suggested by the comparative analysis. They further indicate that which morph is expressed is a function of body design factors discussed in the model. (13) Protonephridia are believed to be primitive to metanephridia because they occur in presumably primitive animals and in ontogenetic stages of many animals with metanephridia as adults. It is suggested here that the distribution of protonephridia is related to small body size and the lack of blood vessels, regardless of phylogenetic status. The occurrence of protonephridia in the larvae of species with metanephridia as adults is explained similarly as a function of the small larval size and lack of blood vessels.     

Developmental morphology of the flower of <Emphasis Type="Italic">Dracontium polyphyllum</Emphasis> in the context of the phylogeny of the Araceae

The inflorescence of Dracontium polyphyllum consists of 150 – 300 flowers arranged in recognisable spirals. The flower has 5 – 6 (90% of observed specimens), or 7 broad tepals enclosing 9 – 12 stamens (occasionally 7) inserted in two whorls. The gynoecium is trilocular (90% of observed specimens) or tetralocular. The tetralocular gynoecia are found at random among the trilocular gynoecia. Each locule encloses an ovule inserted in an axile position, in the median portion of the ovary. Each carpel has its own stylar canal. However, in the upper portion of the style, there is only one common stylar canal. Floral organs are initiated in an acropetal direction in the following sequence: tepals, stamens, and carpels. During later stages of development, the tepals progressively cover the other floral organs. The first floral primordia are initiated on the upper portion of the inflorescence. During early stages of development, the floral primordia have a circular shape. The tepals are initiated nearly simultaneously. During later stages of development, the first whorl of stamens develops in alternation with the tepals and is followed by a second whorl of stamens. The trilocular or tetralocular nature of the ovary is clearly visible during early stages of development of the gynoecium. Recent molecular studies show that Anaphyllopsis A. Hay and Dracontium L. are closely related. However, although pentamerous flowers have been observed in Anaphyllopsis, the developmental morphology of the flower of Dracontium is different from that of Anaphyllopsis.     

Early development of metanephridia in the caudal budding zone of a clitellate annelid, Dero digitata (Naidida): an electron-microscopical study

Ultrastructural analysis has revealed that metanephridia in Dero digitata arise from three nephroblast cells in the frontal epithelium of a septum suggesting its mesodermal origin. Each cell has a fixed developmental destination, one nephroblast cell produces the entire canal part and two cells give rise to the nephrostome. The nephroblast cell nearest to the body wall enlarges and proliferates a first set of canal cells, then one of the two proximally adjacent nephroblast cells differentiates into the envelope of the nephrostome generating the marginal cilia of the opening (mantle cell) and the second one transforms into the anteriormost cell of the funnel, producing a flame of cilia that beats into the canal lumen (flame cell). Thereafter, new canal cells appear, mainly by mitosis of the first cell, enlarging the body of the nephridium whose further differentiation was not analysed. Comparison with other clitellate species suggests a mantle cell (or some marginal cells) and a flame cell (or a central cell) to be special characters of the metanephridium in the stem species of the Clitellata and that, compared to many polychaete species, its early development assumes a special course by a precocious determination and arrangement of nephroblast cells, which, in both groups, probably originate from an identical mesodermal stem cell. Results further indicate that the nonclitellate Aeolosomatidae, by virtue of corresponding nephrostomata, are possibly closer related to the Clitellata.     

The Ontogeny of the Nephridial System of the Larval Amphioxus (Branchiostoma lanceolatum)

Three developmental stages of Branchiostoma lanceolatum were examined by means of transmission electron microscopy. The development of the protonephridium-like cyrtopodocyte from nearly undifferentiated (ento-) mesodermal cells is demonstrated. The ultrastructure of Hatschek's nephridium in an early larval stage is described. The existence of a second filtralional barrier around the rod-like microvilli of the cyrtopodocytes was confirmed. The mesodermal nephridium drains via an excretory canal which is possibly of ectodermal origin into the oral cavity. Cytotic vesicles in the canal cells suggest that the organ is functional in the earliest larval stages. The phylogenetic interpretation of the cyrtopodocyte is clarified as an autapomorphy of the acraniates derived from a podocyte with an apical cilium. The whole system is comparable to the pronephros of craniates and therefore represents a modified metanephridium.     

Mechanisms of ectodermal organogenesis

All ectodermal organs, e.g. hair, teeth, and many exocrine glands, originate from two adjacent tissue layers: the epithelium and the mesenchyme. Similar sequential and reciprocal interactions between the epithelium and mesenchyme regulate the early steps of development in all ectodermal organs. Generally, the mesenchyme provides the first instructive signal, which is followed by the formation of the epithelial placode, an early signaling center. The placode buds into or out of the mesenchyme, and subsequent proliferation, cell movements, and differentiation of the epithelium and mesenchyme contribute to morphogenesis. The molecular signals regulating organogenesis, such as molecules in the FGF, TGFbeta, Wnt, and hedgehog families, regulate the development of all ectodermal appendages repeatedly during advancing morphogenesis and differentiation. In addition, signaling by ectodysplasin, a recently identified member of the TNF family, and its receptor Edar is required for ectodermal organ development across vertebrate species. Here the current knowledge on the molecular regulation of the initiation, placode formation, and morphogenesis of ectodermal organs is discussed with emphasis on feathers, hair, and teeth.     

Ventral nerve cord in Phoronopsis harmeri larvae

The nervous system organization is considered a phylogenetically important character among metazoans. The phylum Phoronida is included in a supraphyletic taxon known as Lophotrochozoa. Many lophotrochozoans possess a metameric ventral nerve cord as adults or larvae. Phoronids do not exhibit external metamery either as larvae or as adults. The current study describes the ventral nerve cord in the young larva of Phoronopsis harmeri. This structure is apparent both in the serotonergic and FMRF-amidergic nervous system in young larvae. The ventral nerve cord extends from the mouth to the tentacular ridge. Both serotonergic and FMRF-amidergic components consist of two ventrolateral nerves, each with several unipolar neurons. The ventrolateral nerves connect to each other by means of thin repetitive transversal nerves ("commissures"). The abundance of neurons and nerves in the epidermis of the oral field of actinotrocha larva likely reflects the importance of this area in collection of food particles. The ventral nerve cords of the actinotrocha and the metatrochophore differ in their positions with respect to ciliated bands: the cord is located between the preoral and postoral ciliated bands in the actinotrocha but between the postoral ciliated band and telotroch in the metatrochophore. The presence of the ventral nerve cord, which contains repetitive elements (neurons and "commissures"), in the early development of P. harmeri may recapitulate some stages of nervous system development during phoronid phylogeny. The larval nervous system does not contain nervous centers under the tentacular ridge that can correlate with the catastrophic metamorphosis and unique body plan of phoronids.     

Ultrastructure of male accessory glands in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae)

The ultrastructure of male reproductive accessory glands was investigated in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae) using light and transmission electron microscopy. The male accessory glands comprise one pair of mesodermal glands (mesadenia) and six pairs of ectodermal glands (ectadenia). The former opens into the vasa deferentia and the latter into the ejaculatory sac. The mesadenia consist of a mono-layered elongated columnar epithelium, the cells of which are highly microvillated and extrude secretory granules by means of merocrine mechanisms. The epithelium of ectadenia consists of two types of cells: the large secretory cells and the thin duct-forming cells. These two types of cells that join with a cuticular duct constitute a functional glandular unit, corresponding to the class III glandular cell type of Noirot and Quennedey. The cuticular duct consists of a receiving canal and a conducting canal. The secretory granules were taken up by the receiving canal and then plunged into the lumen through the conducting canal.     

Ultrastructure of the protonephridia of larval Rugiloricus cf. cauliculus, male Armorloricus elegans, and female Nanaloricus mysticus (Loricifera)

The protonephridial system of several Loricifera was studied by transmission electron microscopy. A larval specimen of Rugiloricus cf. cauliculus possesses two protonephridia, which are "capped" frontally by a compact mass of still undifferentiated gonadal cells. Each protonephridium consists of four monociliary terminal cells and four canal cells with a diplosome but no cilia. Because of incomplete series of sections and unsatisfactory fixation, the outleading cell(s) could not be detected. In a male specimen of Armorloricus elegans, each gonad contains two protonephridia that open into the gonadal lumen. Each protonephridium consists of two monociliary terminal cells, each forming a filter, two nonciliated canal cells, and two nephroporus cells. The protonephridial lumina of the latter cells fuse to one common lumen, which unites with the gonadal lumen. Preliminary observations on the protonephridia of a female Nanaloricus mysticus reveal a more complicated arrangement of interdigitating terminal and canal cells. One or two terminal cells form their own individual filter or four cells form a common compound filter. The cilium of the terminal cells of all species investigated are surrounded by a palisade of nine microvilli that support the filter barrier made of an extracellular matrix. An additional filter diaphragm could be traced between the pores in the cell wall of each terminal cell of A. elegans. The urogenital system of the Loricifera differs from that of the Priapulida in that the protonephridia of the former are completely integrated into the gonad, whereas the excretory organs of the latter open into the urogenital duct caudally of the gonads.     

Initial limb budding is independent of apical ectodermal ridge activity; evidence from a limbless mutant

Outgrowth of normal chick limb bud mesoderm is dependent on the presence of a specialized epithelium called the apical ectodermal ridge. This ectodermal ridge is induced by the mesoderm at about the time of limb bud formation. The limbless mutation in the chick affects apical ectodermal ridge formation in the limb buds of homozygotes. The initial formation of the limb bud appears to be unaffected by the mutation but no ridge develops and further outgrowth, which is normally dependent on the ridge, does not take place. As a result, limbless chicks develop without limbs. In the present study, which utilized a pre-limb-bud recombinant technique, limbless mesoderm induced an apical ectodermal ridge in grafted normal flank ectoderm. However, at stages when normal flank ectoderm is capable of responding to ridge induction, limbless flank ectoderm did not form a ridge or promote outgrowth of a limb in response to normal presumptive wing bud mesoderm. We conclude from this that the limbless mutation affects the ability of the ectoderm to form a ridge. In addition, because the limbless ectoderm has no morphological ridge and no apparent ridge activity (i.e. it does not stabilize limb elements in stage-18 limb bud mesoderm), the limbless mutant demonstrates that the initial formation of the limb bud is independent of apical ectodermal ridge activity.     

Alimentary canal formation in the stonefly,Kamimuria tibialis (pictet) (Plecoptera : Perlidae)

The alimentary canal formation in the stonefly, Kamimuria tibialis (Plecoptera : Perlidae) is described. The stomodaeum is formed as in other insect embryos. The proctodaeum is derived from the ectodermal fold an the caudal end of the embryo without the contribution of the amnion. The 3 Malpighian tubules develop from the blind end of the proctodaeum. The rectal pad is formed by the thickening of the dorsal wall of the proctodaeum. The midgut epithelium rudiment arises only from the blind end of the proctodaeum, i.e. it is completed by unipolar formation instead of bipolar. The yolk cells do not contribute to the formation of the midgut epithelium. The alimentary canal is transformed during the 1st nymphal instar and becomes functional in the next instar. The stomodaeum is differentiated into 3 parts: pharynx, oesophagus, and proventriculus. The midgut becomes shortened and its epithelium is well developed. Gastric caeca with tapering processes are formed.     

Differential gene expression in the jellyfish Aurelia aurita

The body of Aurelia aurita, as well as other diploblasts, consists of two epithelial layers: ectodermal and gastral epithelium. These two tissues are separated by mesoglea, or extracellular matrix. In most coelenterates mesoglea is acellular. In A. aurita mesogleal cells are scattered in mesoglea. Differential display PCR was used to compare mRNA pools from ectodermal epithelium, gastral epithelium and mesoglea. 4 novel gene fragments were cloned and sequenced. According to RTPCR results, one of these fragments is differentially expressed in the ectodermal epithelium.     

Ultrastructural analysis of the apical ectodermal ridge during vertebrate limb morphogenesis: I. The human forelimb with special reference to gap junctions

The fine structure of the human forelimb apical ectodermal ridge of stages 12–19 was examined using techniques of transmission electron microscopy, freeze fracture, and scanning electron microscopy. This paper reports the presence of subcellular structures that distinguish the inductively active apical ectoderm from adjacent dorsal and ventral ectoderms.The apex of the human forelimb begins development with an epithelium of two cell layers (stage 12) which thickens at the distal tip during stages 13 and 14 into a multilayered apical ectodermal ridge. During this transition we have observed that the basal lamina differentiates from a bilayered structure to the definitive single lamina. Some cells in the ectoderm become detached from the basal lamina as stratification begins. At the same time these cells show increased mitotic activity and the developing ridge cells acquire gap junctions. Annular gap junctions are also observed. Gap junctions are not observed in adjacent, presumably noninductive, epithelia. Finally, the ridge cells next to the basal lamina acquire bundles of microfilaments that are oriented in the dorsal-ventral plane in the basal cytoplasm of the cells.The apical ridge reaches its greatest dimensions during stage 15. The number and peripheral extent of gap junctions also appear to be greatest at this same time. At stage 17, cells within the ridge begin to die, and other ridge cells engulf them. By stage 19, gap junctions in the apical epithelium are sparse and are of lesser diameter than in the definitive ridge. In addition, the oriented bundles of microfilaments present at stages 14–17 are absent. Thus, at stage 19 a morphologically distinct apical ectodermal ridge is no longer present. The apex of the limb is covered by two cell layers typical of human embryonic epidermis.     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Histogenesis of Swiss white mouse secondary palate from nine and one-half days to fifteen and one-half days In utero. I. Epithelial-mesenchymal relationships—light and electron microscopy

Development of the secondary palate in Swiss white mouse embroyos was studied from age nine-and-one-half days in utero to the stage of mesenchymal coalescence in the secondary palate (approximately fifteen-and-one-half days). The greatest changes observed occur in the mesenchyme. At early stages, mesenchymal cells underlying oral ectoderm of the head are few and only occasionally contact the ectoderm. Electron micrographs show large intercellular spaces between the ectodermal cells. As embryogenesis continues, the mesenchymal cells become more numerous, closer to each other and closer to the epithelium. Just prior to horizontal transposition of shelves, the mesenchymal cells spread farther from each other and from the palatal epithelium and epithelium of the palatal tip becomes stretched. Ultrastructurally the intercellular spaces between epithelial cells of the palate tip have become much smaller. Some mitochondria in some epithelial cells are swollen and have clear matrices and distorted cristae. The shelves become horizontal and meet in the midpalate. Cells with degeneration bodies are seen in the epithelial seam. The seam undergoes autolysis and is replaced by mesenchyme. The morphological changes described, particularly in the mesenchyme, may play an important role in determining the effect of various teratogens at different stages of palatal development. The changes in both mesenchyme and epithelial cells in the later stages may constitute part of the process of preparing shelves for fusion as postulated by Pourtois ('66).