In the bug, Pyrrhocoris apterus, blastokinesis (a reversal of the position of the embryo within the egg) is seen to involve contraction of the serosa that is attached to the embryo’s head. As the serosal cells change from squamous to columnar in the course of blastokinesis, a dense zone of microfilaments appears just under the apical surface. Many apical protrusions develop above this zone. After the embryo is in its final position the zone disappears and later the cells degenerate.

Laterally, the serosal cells are connected by belt desmosomes, septate junctions and gap junctions. As blastokinesis progresses, more lateral surface is recruited below them from the original basal surface.

Microtubules running parallel to the plasma membrane are seen near the apical microfilaments and along other surfaces of the cell. Secretory granules are evident both within serosal cells and along the apical surface, probably providing a lubricant for movement against the chorion. Yolk cells are common basal to the serosa, possibly mobilizing nutrients for it.

This study of blastokinesis in Pyrrhocoris provides a dramatic example of cell shape change that is correlated with the appearance of microfilaments. In its details blastokinesis is comparable to morphogenetic events such as amphibian neural tube formation and ascidian metamorphosis.

Blastokinesis (katatrepsis phase) results in a reversal of the insect embryo’s position within the egg. Recent studies of Hemiptera have indicated that this change involves the pulling of the embryo by a sheet of epithelial cells, the serosa, which is attached to the head of the embryo (Cobban, 1968; Enslee & Riddiford, 1977). During blastokinesis, this squamous epithelium contracts into a small knob of columnar cells. This has been observed in numerous hemipterans (Butt, 1949; Cobban, 1968; Mellanby, 1936; Seidel, 1924). A contractile pulling function was proposed for the serosa-over a century ago (Brandt, 1869, cited by Sander, 1976), but no attempt to relate cell structure and function has appeared in the literature. In other orders of insects other parts of the egg, especially the embryo, have been implicated, reflecting the different organization of the eggs studied (reviewed by Enslee & Riddiford, 1977).

The following light and electron microscopic study of blastokinesis in embryos of the linden bug, Pyrrhocoris apterus, indicates the appearance of an apical zone of microfilaments in the serosal cells during this process. The relationship of this zone to adjacent apical and lateral cell surfaces is consistent with a contractile role. This contraction would account for both the pulling action of the serosa and the dramatic change in the shape of its constituent cells, comparable to what has been found in many other systems (Baker & Schroeder, 1967; Burnside, 1971; Cloney, 1966; Pollard & Weihing, 1974; Schroeder, 1976; WessellseZ al., 1971).

For collection of timed batches of fertilized eggs, mating pairs of Pyrrhocoris adults were kept under an 18 L:6D photoperiod at 30·5 °C, 60–70% relative humidity (Enslee & Riddiford, 1977). Eggs at appropriate developmental stages were selected by means of a brief immersion in 95 % ethanol which rendered the chorion transparent. This treatment had no adverse effect on subsequent embryonic development. For both light and electron microscopy, each stage was represented by at least three individuals.

The eggs were dechorionated and then fixed in 3 % glutaraldehyde for light microscopy serial sections as described previously (Enslee & Riddiford, 1977) Thick sections (1–2 μm) of the material prepared for electron microscopy were used for extensive supplemental observations. For electron microscopy, eggs were fixed at room temperature according to the glutaraldehyde-H2O2 method of Perrachia & Mittler (1972) using 6% glutaraldehyde in 2 mM-CaCl2, 0·1 M sucrose and 0·1 M phosphate buffer (pH 7·4). For rapid fixation the chorion of the submerged egg was pierced with a minutien and the hole was quickly and carefully enlarged. As soon as the tissue surfaces were slightly rigid, the chorion was removed. Final dissection and gross observation were done in the buffer wash (2 mM-CaCl2, 0·616 M sucrose, 0·1 M phosphate buffer, pH 7·4) just prior to the osmium tetroxide fixation step (2% OsO4 in 0·1 M phosphate buffer, pH 7·4 with 2 mM CaCl2. Just before dehydration the eggs were stained en bloc with 0·5% urahyl magnesium acetate in 0·9% NaCl for 2h. Then they were embedded in Epon.

Sections were taken from the anterior polar region, in the median plane. Some near-tangential sections were also prepared. Thin sections were stained with 7·5% uranyl magnesium acetate for 3 h (Frasca & Parks, 1965) and with 0·35% lead citrate for four minutes (Reynolds, 1963). All micrographs are of double-fixed Epon-embedded specimens. Toluidine blue staining with phasecontrast optics and an orange (No. 23A) Wratten filter was used for photomicrographs and a Siemens Elmiskop I for electron micrographs.

Changes in the serosal cells during blastokinesis

The changing positions of the Pyrrhocoris embryo and serosa during blastokinesis are diagrammed in Fig. 1 (Enslee & Riddiford, 1977). The serosa changes from a squamous (Fig. 2 A) to a low columnar epithelium (Fig. 2B) as the embryo’s head moves to the anterior pole. When the head is crossing the anterior pole, the serosal cells are about five times as high as wide (Fig. 2C). When the head is in its final position, the serosa invaginates into the yolk with its cells becoming flask-shaped (Fig. 2 D). In this form it is called the dorsal organ (Cobban, 1968). Shortly afterward, the lateral surfaces of the embryo come together in the dorsal midline (dorsal closure), and the dorsal organ degenerates.

Fig. 1.

Semidiagrammatic longitudinal sections through embryos, showing changes in position as blastokinesis progresses. Embyo (A) is typical of a stage shortly before the process begins. Embryo (B) is turning around. Embryo (C) has reached its final position. The anterior pole of the egg is always at the top. Eggs are approximately 1 mm in length, ab = abdomen; am = amnion; ap = appendage; asb = amnionserosa border; hd = head; s = serosa; yk = yolk. The germ band (embryo) is stippled. (This figure is adapted from one in Enslee & Riddiford (1977). Permission has been obtained from the publishers to reprint it.)

Fig. 1.

Semidiagrammatic longitudinal sections through embryos, showing changes in position as blastokinesis progresses. Embyo (A) is typical of a stage shortly before the process begins. Embryo (B) is turning around. Embryo (C) has reached its final position. The anterior pole of the egg is always at the top. Eggs are approximately 1 mm in length, ab = abdomen; am = amnion; ap = appendage; asb = amnionserosa border; hd = head; s = serosa; yk = yolk. The germ band (embryo) is stippled. (This figure is adapted from one in Enslee & Riddiford (1977). Permission has been obtained from the publishers to reprint it.)

Fig. 2.

Light micrographs of serosa (A) just prior to and (B) about midway through blastokinesis; (C) as the head of the embryo is moving across the anterior pole of the egg and (D) as the serosa (now the dorsal organ) sinks into the yolk. Osmiophilic yolk granules, clear vesicles and nuclei are visible in the cells. A dense apical zone (bracketted in B) is present after blastokinesis begins. Bar 50 μm. d = degenerating cells; gb = germ band; s = serosa; y = yolk mass; yc -yolk cell; * = location of Fig. 8.

Fig. 2.

Light micrographs of serosa (A) just prior to and (B) about midway through blastokinesis; (C) as the head of the embryo is moving across the anterior pole of the egg and (D) as the serosa (now the dorsal organ) sinks into the yolk. Osmiophilic yolk granules, clear vesicles and nuclei are visible in the cells. A dense apical zone (bracketted in B) is present after blastokinesis begins. Bar 50 μm. d = degenerating cells; gb = germ band; s = serosa; y = yolk mass; yc -yolk cell; * = location of Fig. 8.

Throughout blastokinesis the cytoplasm of serosal cells is metachromatic. The nuclei have conspicuous nucleoli and little heterochromatin. Osmiophilic yolk granules and clear vesicles are common features of these cells. As the cells become columnar, the yolk granules are found more often in the basal halves of the cells whereas small clear vesicles are more common just apical to the centrally located nuclei (Fig. 2C). A new population of giant clear vesicles dominates the mid-region of the cells after dorsal-organ formation (Fig. 2D).

With phase-contrast optics, a smooth dense line just below the apical surface of the serosa is evident during blastokinesis. (Fig. 2 B). Above this line the surface of the cells is ruffled. The line is sharpest as the embryo nears the anterior pole; then as the head moves across the pole and the serosa invaginates, interruptions and ripples are seen in this zone (Fig. 2C, D). In order to elucidate more clearly these changes, it was necessary to turn to a study of the ultrastructure of the cells.

Fine structure of the serosa during blastokinesis

1. Preblastokinesis

Just prior to blastokinesis the serosal cells are flat. Patches of punctate or filamentous material are sometimes found in the apical cytoplasm (Fig. 3 A). Microtubules lie parallel to all cell surfaces.

Fig. 3.

Electron micrographs of serosa just prior to blastokinesis, showing adjacent cell surfaces. (A) Gap junctions are sometimes found. (B) Near the apical surface is a belt desmosome, basally, a septate junction. Breaks in membrane apparently occur during removal of closely appressed chorion. Bar 0·5 μm. bd = belt desmosome; gj = gap junction; mt = microtubule; s = serosal cell; sj = septate junction; y = yolk cell.

Fig. 3.

Electron micrographs of serosa just prior to blastokinesis, showing adjacent cell surfaces. (A) Gap junctions are sometimes found. (B) Near the apical surface is a belt desmosome, basally, a septate junction. Breaks in membrane apparently occur during removal of closely appressed chorion. Bar 0·5 μm. bd = belt desmosome; gj = gap junction; mt = microtubule; s = serosal cell; sj = septate junction; y = yolk cell.

Adjacent cells are connected by three organelles. Near the apical surface is a belt desmosome, with parallel membranes thickened on their cytoplasmic surfaces and very thin intercellular bridges. Basal to the belt desmosomes is a zone of septate junctions and occasional gap junctions (Fig. 3 A, B). The desmo-some and junctional region are often convoluted and sometimes interrupted by a vesicle-like intercellular space. (Junction terminology conforms to Satir & Gilula, 1973).

2. Blastokinesis

During blastokinesis the apical surface became increasingly ruffled above the dense line seen with the light microscope (Fig. 4 A, B). These cytoplasmic protrusions sometimes contain organelles such as mitochondria and rough endoplasmic reticulum (RER). The dense zone itself is composed of two components: (1) a layer of microfilaments and microtubules running parallel to the egg’s surface; and (2) convolutions in the belt desmosome on the cell’s lateral plasma membranes (Fig. 4B). The folds of these lateral membranes nearest the filamentous layer are distinctive in that they are almost parallel to the surface. The microfilaments appear to merge with the lateral membranes or their dense cytoplasmic sides and also criss-cross the bases of apical protrusions. To ascertain the orientation of microtubules and microfilaments within the planar apical zone, sections were cut nearly tangential to the serosal surface. Some of the microtubules are aligned parallel to, or curved along, a lateral margin while others appear to lie randomly (Fig. 5). Microfilaments sometimes occur in bundles or lie in one dominant direction (Fig. 5), but no constant pattern was evident. Perfectly tangential sections could not be obtained due to the large size of the cells and the curvature of the egg but examination of many slightly oblique sections indicated that microfilaments are present throughout most of the cell apex. Possibly their density is variable.

Fig. 4.

Electron micrographs of serosal cells when embryo’s head was about three-fourths of the way toward anterior pole (corresponds to Fig. 2B). (A) Overview, (B) closeup of apical protrusions and apical zone of microfilaments, microtubules and belt desmosomes (arrows in (A)). Bar 1·0 μm. ap = apical protrusions; bd = belt desmosome; f = extracellular flocculent material; yg = yolk granule; circles denote microtubules.

Fig. 4.

Electron micrographs of serosal cells when embryo’s head was about three-fourths of the way toward anterior pole (corresponds to Fig. 2B). (A) Overview, (B) closeup of apical protrusions and apical zone of microfilaments, microtubules and belt desmosomes (arrows in (A)). Bar 1·0 μm. ap = apical protrusions; bd = belt desmosome; f = extracellular flocculent material; yg = yolk granule; circles denote microtubules.

Fig. 5.

Section nearly tangential to the surface of a serosal cell in an egg where the embryo’s head is about half-way to the anterior pole. Both parallel and randomly oriented microtubules and microfilaments are present. Bar 1·0 μm. ap = apical protrusion; mf = microfilaments: arrows denote microtubules.

Fig. 5.

Section nearly tangential to the surface of a serosal cell in an egg where the embryo’s head is about half-way to the anterior pole. Both parallel and randomly oriented microtubules and microfilaments are present. Bar 1·0 μm. ap = apical protrusion; mf = microfilaments: arrows denote microtubules.

The junctional zone continues to be prominent as the cells become more cuboidal. However, extensive non-junctional surface appears laterally, and the basal surface area decreases. No distinct boundary separates the basal and the non-junctional lateral surfaces. The spaces between adjacent cells are irregular and filled with a flocculent material (Fig. 4A).

In contrast to cell-surface configurations, the internal cytoplasmic organelles undergo no dramatic changes during blastokinesis. The RER appears to become more abundant and is occasionally seen in stacked cisternae, especially late in the process. At all stages small granules, about 70 to 110 nm in diameter, are seen throughout the cell in Golgi complexes and other vesicles (Fig. 6), and on the apical surface of the cell, particularly among the protrusions (Fig. 4B). Microtubules are common near the plasma membrane, tending to be parallel to it, not exclusively in the apical zone.

Fig. 6.

Section through the serosa late in blastokinesis showing intercellular zones below the septate junctions. Bar 1·0 μm. yg = yolk granule; sec = vesicle with secretory granules.

Fig. 6.

Section through the serosa late in blastokinesis showing intercellular zones below the septate junctions. Bar 1·0 μm. yg = yolk granule; sec = vesicle with secretory granules.

3. Late blastokinesis

As the head starts to traverse the pole, the serosal cells become laterally compressed (Fig. 2C). The junctions between cells, perpendicular to the egg surface, become much longer. Below the often convoluted septate junction, the facing membranes appear scalloped (Fig. 6). Most of the cell surface is now lateral as the basal surface has been shifted up and laterally. The apical surface is ruffled above a somewhat less distinct zone of microfilaments and microtubules. In one-quarter to one-half of the sampled cells of a typical individual undergoing blastokinesis, a zone that contains only free ribosomes is found either lateral or basal to the nucleus; when the head is crossing the pole, it appears to be only basal. In one egg, where the embryo’s head was about three-fourths of the way towards the anterior pole, filamentous material was seen in this region (Fig. 7).

Fig. 7.

Slightly oblique section showing the base of the serosa in an egg where the embryo’s head was three-fourths of the way to the anterior pole. A zone of free ribosomes (r) is evident. Small arrows mark a filamentous mat. Large arrows denote the overall orientation of the base (b) of the serosa. Bar 2·0 μm. f = extracellular flocculent material; s = serosal cell; yc = yolk cell.

Fig. 7.

Slightly oblique section showing the base of the serosa in an egg where the embryo’s head was three-fourths of the way to the anterior pole. A zone of free ribosomes (r) is evident. Small arrows mark a filamentous mat. Large arrows denote the overall orientation of the base (b) of the serosa. Bar 2·0 μm. f = extracellular flocculent material; s = serosal cell; yc = yolk cell.

4. Formation of the dorsal organ

When the embryo’s head is in position covering the pole of the egg, the serosa assumes a concave form as it invaginated to form the dorsal organ. Its flaskshaped cells (Fig. 2D) are still connected in their apical halves by the convoluted lateral zone, including belt desmosomes and septate junctions. An apical band is distinctly seen by phase-contrast optics, but not in thin sections, possibly because the extreme concavity of the apical surface precludes a section ideal for displaying the components. Some of the giant clear vesicles seen in mid-cell in Fig. 2D are found to be invaginations of extra-cellular space. Among and basal to the flask-shaped cells are found disintegrating cells (Fig. 8). Somewhat later during dorsal closure of the embryo, this dorsal organ region is occupied by more-or-less spherical cells showing abundant evidence of degeneration – pycnotic nuclei, swollen RER, dense inclusions, and membranous whorls (Fig. 9). In some, the cytoplasm is very condensed. Intercellular associations and structural asymmetry are lacking or diminished. Adjacent to these cells are various healthy cells of the developing embryo.

Fig. 8.

Basal portions of bottle-shaped serosal cells (s) in an invaginated dorsal organ. Degenerating cells (dc) are present within and outside of the dorsal organ in the yolk cell (yc) region. Arrows denote general orientation of bases (b) of serosal cells. Bar 10 μm. dse = dorsal surface of the embryo.

Fig. 8.

Basal portions of bottle-shaped serosal cells (s) in an invaginated dorsal organ. Degenerating cells (dc) are present within and outside of the dorsal organ in the yolk cell (yc) region. Arrows denote general orientation of bases (b) of serosal cells. Bar 10 μm. dse = dorsal surface of the embryo.

Fig. 9.

Degenerating serosal cells in an embryo which is completing dorsal closure. The loss of asymmetry is a prominent feature of these cells. Bar 10 μm. pn = pycnotic nucleus.

Fig. 9.

Degenerating serosal cells in an embryo which is completing dorsal closure. The loss of asymmetry is a prominent feature of these cells. Bar 10 μm. pn = pycnotic nucleus.

Yolk cells

Yolk cells are defined as any cell observed near the serosa but never in direct contact with the egg surface and never joined with a serosal cell by a septate junction or belt desmosome. Two types of yolk cells were observed: (1) a thin cell which extends long and sometimes convoluted processes along the basal surface of the serosa; it is a constant feature in pre-or early blastokinesis (Fig. 3B); (2) a thicker cell whose cytoplasm displaces the large osmiophilic yolk granules (Fig. 2C). This latter cell type is often seen near the tip of the abdomen before and during early blastokinesis. Then, as blastokinesis progresses several are usually found directly under the thickening serosa (Fig. 2C). In electron micrographs these cells display a distinctive fenestrated appearance (Figs. 4A, 7 and 8). The internalized membranes and spaces are judged to represent surface on the basis of the similarity of flocculent material in them and in the intercellular spaces; however, the surface of the yolk cell immediately below the serosa rarely appear to have invaginations. Thus, the continuity of the internal and external zones is uncertain. Mitochondria and RER are common in the cytoplasmic trabeculae, and a Golgi apparatus is sometimes seen.

During blastokinesis, the spaces between the serosa and the yolk cells vary from broad to very narrow, even in the same embryo. By the time the head is moving across the pole, serosa and yolk cells are packed closely together (Fig. 7). At no time is a basal lamina or other extracellular fibrous material visible.

A contractile function for the serosa

The following observations of our ultrastructural study are consistent with the hypothesis that the serosa is an active participant in moving the embryo through blastokinesis.

  • An apical zone of microfilaments becomes conspicuous in serosal cells when the head of the embryo is moving from the posterior toward the anterior pole of the egg (see Fig. 10). The zone is remarkably taut, parallel to the surface of the egg.
    Fig. 10.

    Diagrammatic representation of changes in the serosal cell during blastokinesis. Arrows denote location of diagrammed cells. Magnification of the two cells is the same except that the maximum width of the preblastokinesis cell would probably be greater than what is shown. As the cell elongates, the apical surface is thrown into protrusions (up) above an apparently contractile band of microfilaments (mf). Cells are held together by a lateral zone consisting of a belt desmosome (bd), a septate junction (sj) and gap junctions (not shown).

    Fig. 10.

    Diagrammatic representation of changes in the serosal cell during blastokinesis. Arrows denote location of diagrammed cells. Magnification of the two cells is the same except that the maximum width of the preblastokinesis cell would probably be greater than what is shown. As the cell elongates, the apical surface is thrown into protrusions (up) above an apparently contractile band of microfilaments (mf). Cells are held together by a lateral zone consisting of a belt desmosome (bd), a septate junction (sj) and gap junctions (not shown).

  • Above this zone the apical surface is thrown into many protrusions.

  • Microtubules are seen parallel to the surfaces of the cell at all stages.

  • Laterally the cells are joined by belt desmosomes and septate and gap junctions. Prior to blastokinesis, the entire lateral surface consists of these specialized zones. During blastokinesis, unspecialized basal surface is shifted laterally to a position below the junctional zones.

These features of the serosa bear a striking resemblance to several other systems. During tail resorption in certain ascidians, cells of the tail epidermis become contractile. These cells develop a zone of apical microfilaments which are oriented primarily in the axis of contraction – parallel to the tail axis (Cloney, 1966). In the folding neural plate of amphibians (Baker & Schroeder, 1967; Burnside, 1971), morphogenesis is correlated with the tapering of the constituent cells as the microfilaments appear in a ring in the cells’ apices. Some of the serosal microfilaments of Pyrrhocoris run in parallel arrays and others describe small arcs. Some are seen in the bases of apical protrusions as in the ascidian tail epithelium. The protrusions are consistent with the existence of very small, irregular fields in the apical surface, delimited by contractile elements which are connected to the plasma membrane. The serosal cell apex may be covered with such fields, each microfilament bundle being a side of one field or of two adjacent fields. After contraction had occurred, an apical protrusion would have bulged upward as the field perimeter narrowed around its base.

The apical protrusions seen in the amphibian cells are not very conspicuous. The ascidian tail epithelial cells, where the microfilament zone is throughout the apex, develops many protrusions like those in the serosa, except that they are somewhat more elongate.

Desmosomes are commonly associated with microfilaments and apparently serve as anchoring sites (Satir & Gilula, 1973 ; Burnside, 1971). In the Pyrrhocoris embryos a fold in the serosal belt desmosome usually is oriented in line with the apical microfilament zone. Thus the microfilaments seem to be actively pulling against it.

Additional support for the intrinsic contractile nature of the serosa comes from the cases of juvenile-hormone-exposed embryos described by Enslee & Riddiford (1977). In such embryos the serosa commonly breaks during blastokinesis but contracts anyway, even though the embryo does not complete blastokinesis. Thus the embryo is not compressing it.

But contraction of the apical microfilaments of the serosa may not be the only force pulling on the embryo, especially in the later stages of blastokinesis. The amnion or the amnion-serosa junction may be especially important then. Invagination, which the serosa undergoes at the end of blastokinesis, is the expected result of continued apical contraction (Lewis, 1947). But if only the cell apices were contracting, the invagination would occur at the centre of the anterior pole and the head of the embryo would not move across the pole. Instead, the head does move across the pole, changing its position relative to the extraembryonic membranes (Enslee & Riddiford, 1977). A band of filamentous material was detected in the basal region of a serosal cell as the head approached the pole (Fig. 7), but its role and that of the amnion-serosa junction in these complex movements remain to be clarified.

Microtubules appear to maintain a constant relationship to the plasma membrane throughout blastokinesis and probably serves a cytoskeletal function. Apically they always were associated with the microfilaments which may indicate an interaction between the two during movement.

The septate and gap junctions seen between the serosal cells are indicative of intercellular communication (Satir & Gilula, 1973; Lowenstein, Kanno & Socolar, 1978). Combined with desmosomes, these are typical of insect epidermis (Caveny, 1976; Poodry & Schneiderman, 1970), follicle cells (Mahowald, 1972), and salivary glands (Oschman & Berridge, 1970; Lowenstein, 1975); hence, it is not surprising to find them in this important extraembryonic membrane as well.

The terminal stage of blastokinesis is marked by cell death in the serosa (Figs. 8 and 9). It is likely, then, that some of the synthetic activity evident in the serosa cells (active nucleus and abundant polyribosomes) was directed toward forming hydrolytic enzymes to be activated when the cells had carried out their primary function (Novikoff, Essner & Quintana, 1964; Wattiaux, 1969).

The environment of the serosal cell

Secretory granules are found both within and at the apical surfaces of the serosal cells, and flocculent material is seen in the basal and non-junctional lateral regions. The latter may have come from either the serosa or the yolk cells. The function of both of these materials may possibly be lubrication. Since the serosa was moving against the inside of the chorion under considerable pressure of the egg contents, a lubricant would facilitate progress.

Another possible function for the basal flocculent material is nutrition for the serosa. The yolk cells are similar, though not identical, to cells of the perineurium in insect ganglia (Smith, 1967). The perineurium cells have been credited with transporting food to the cells of the ganglion which are remote from the blood (Wigglesworth, 1960). Like the yolk cells they are fenestrated and have processes which they send into the tissue they are feeding. The absence of basal lamina beneath the serosa is consistent with this idea. Johannsen & Butt (1941) assert that the function of the yolk cells, or vitellophages as they are sometimes called, is to transform the yolk so it can be assimilated by the embryo. In the hemipteran, Gerris paludum insularis, these vitellophages have also been recognized as progenitors of the midgut epithelium (Mori, 1976). The absence of a basal lamina is of further interest because in the ascidian tadpole metamorphosis (Cloney, 1966) the basal lamina split off from the epithelial cells at the start of tail resorption, and remained with the underlying muscle cells as the epithelium moved away. In contrast, the basal lamina was gradually forming as neurulation progressed in the amphibian Taricha torosa (Burnside, 1971).

Significance of contractile serosa

The serosa has been shown to consist of contractile cells containing a taut apical zone of microfilaments whose time and site of appearance correspond to the presence of a constricting force. It thus resembles other epithelia in which cells are changing shape (Schroeder, 1976), especially in a short time. Like the epidermis in the collapsing tail of the ascidian tadpole (Cloney, 1966), it is characterized by very rapid and extreme cell shape changes. The ascidian Amaroucium completes tail resorption in about six minutes. The major posterior-to-anterior movement of blastokinesis takes less than an hour (Enslee & Riddiford, 1977). In contrast, the remodelling of the amphibian neural plate requires many hours (Burnside & Jacobson, 1968), and cell shape changes are not so dramatic.

Contraction of microfilament zones is a fundamental process in morphogenesis. In speed and degree of cell shape change, serosal contraction in Pyrrhocoris and ascidian tail resorption appear to represent extreme expressions of the phenomenon. ;

We wish to thank Dr Oscar Auerbach for the use of his electron microscopy facilities and supplies at the Veterans Administration Hospital, East Orange, NJ., and his colleagues, especially Dr Julio Frasca and Terry Parks for invaluable assistance; Dr Christopher Woodcock for technical guidance early in the research; Dr Beth Burnside for photomicroscopy facilities; and Drs Richard Cloney, John Edwards and Peddrick Weis for reading the manuscript.

This research was supported by funds from the Milton Fund, Harvard University and from the National Science Foundation to L.M.R., and Biology department funds at Seton Hall University to E.C.E.

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