Fate maps of the first quartet micromeres in the gastropod Ilyanassa obsoleta.

Cell fate specification in the gastropod mollusc Ilyanassa obsoleta involves both cell autonomous and inductive mechanisms, which depend on determinants localized first in the polar lobe and then in the D quadrant of the embryo. A complete cell lineage is lacking for this embryo and is essential for a critical interpretation of previous experimental results and an analysis of the mechanisms at the molecular level. Lineages of the first quartet micromeres were followed using Lucifer Yellow dextran as a tracer. The tracer was injected into individual first quartet micromeres using iontophoresis and patterns of fluorescence were analyzed in the larva after 8 days of development. Fluorescence was limited to head structures, including eyes, tentacles and velum. Structures on the left side were derived from 1a and 1d micromeres; 1a gave rise to the left eye, including the lens. Right side structures were derived from the 1c micromere and 1b contributed to the apical plate between the eyes and symmetrically to both sides of the velum. First quartet lineage data are compared with results from previous cell ablation experiments and with lineage data from other species.     

Cell specification and the role of the polar lobe in the gastropod mollusc Crepidula fornicata

A small polar lobe forms at the first and second cleavage divisions in the gastropod mollusc Crepidula fornicata. These lobes normally fuse with the blastomeres that give rise to the D quadrant at the two- and four-cell stages (cells ultimately generating the 4d mesentoblast and D quadrant organizer). Significantly, removal of the small polar lobe had no noticeable effect on subsequent development of the veliger larva. The behavior of the polar lobe and characteristic early cell shape changes involving protrusion of the 3D macromere at the 24-cell suggest that the D quadrant is specified prior to the sixth cleavage division. On the other hand, blastomere deletion experiments indicate that the D quadrant is not determined until the time of formation of the 4d blastomere (mesentoblast). In fact, embryos can undergo regulation to form normal-appearing larvae if the prospective D blastomere or 3D macromere is removed. Removal of the 4d mesentoblast leads to highly disorganized, radial development. Removal of the first quartet micromeres at the 8-cell stage also leads to the development of radialized larvae. These findings indicate that the embryos of C. fornicata follow the mode of development exhibited by equal-cleaving spiralians, which involves conditional specification of the D quadrant organizer via inductive interactions, presumably from the first quartet micromeres.     

MAPK activation and the specification of the D quadrant in the gastropod mollusc, Crepidula fornicata

Embryos of the gastropod snail Crepidula fornicata exhibit a typical spiral cleavage pattern. Although a small polar lobe is formed at the first and second cleavage divisions, the embryo of C. fornicata exhibits a mode of development similar to that of equal-cleaving spiralians in which the D quadrant is conditionally specified by inductive interactions involving the derivatives of the first quartet micromeres. This study demonstrates that mitogen activated protein kinases, MAPK, are initially activated in the progeny of the first quartet micromeres, just prior to the birth of the third quartet (e.g., late during the 16-cell and subsequently during the 20-cell stages). Afterwards, MAPK is activated in 3D just prior to the 24-cell stage, transiently in 4d and finally in a subset of animal micromeres immediately following those stages. This pattern of MAPK activation differs from that reported for other spiralians. Using an inhibitor of MAPK kinase (MEK), we demonstrated that activated MAPK is required for the specification of the 3D macromere, during the late 16-cell through early 24-cell stages. This corresponds to the interval when the progeny of the first quartet micromeres specify the D quadrant macromere. Activated MAPK is not required in 3D later during the 24-cell stage or in the embryonic organizer, 4d, for its normal activity. Likewise, activated MAPK is not required in the animal micromeres during subsequent stages of development. Additional experiments suggest that the polar lobe, though not required for normal development, may play a role in restricting the activation of MAPK and biasing the specification of the 3D macromere.     

The ‘organizing’ role of the D quadrant in an equal-cleaving spiralian,Lymnaea stagnalis as studied by UV laser deletion of macromeres at intervals between third and fourth quartet formation

Summary

In the spiralian embryos studied which display unequal-cleavage at the first two cleavages (either by a polar lobe or an asymmetric cleavage mechanism) the D quadrant is determined at the four cell stage by an unequal segregation of cytoplasmic stuffs. The normal formation of eyes, foot, and shell by overlying micromeres in these forms requires the inductive interaction with the D quadrant before the formation of the third quartet of micromeres. In equal-cleaving spiralians the D quadrant (3D macromere) becomes determined as a result of inductive interactions with first quartet derivatives (animal-vegetal interaction) sometime after the production of the third quartet of micromeres. This paper investigates the exact timing of D quadrant determination and the inductive role of third-order macromeres on the development of micromere derived structures in an equal-cleaving spiralian. Deletions of third-order macromeres, and their derivatives, were performed without rupturing the egg capsule membrane of the Lymnaea embryo with a UV laser microbeam. Virtually normal snails were produced when the 3A, 3B, 3C, or 4D macromere was irradiated. Juvenile snails lacking all mesodermal structures but possessing eyes, foot, and shell were obtained when the mesentoblast (4d) or its progenitor (3D) were deleted. Furthermore, ‘mesoderm-less’ snails were produced by deleting one of the two possible 3D candidates (cross furrow macromeres) as early as 20 min after third quartet formation. These results indicate that the 3D macromere begins to become determined at, or soon after, animal-vegetal interaction; before the 3D macromere becomes visibly distinguishable from the 3B macromere. The results also demonstrate that normal pattern formation in the overlying micromeres does not require the ‘prolonged’ interaction with an asymmetrically positioned 3D macromere. Possible adhesive differences between the 3D macromere and the remaining three macromeres are also revealed.     

Conserved mechanism of dorsoventral axis determination in equal-cleaving spiralians

Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal-vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.     

The embryonic value of the micromeres in Ilyanassa obsoleta,as determined by deletion experiments. II. The second quartet cells

Summary

Each of the second quartet micromeres of Ilyanassa obsoleta was removed and the effects on larval development analyzed. Structures most often affected by removal of 2a were the left velar lobe, the left eye and the left statolith. Removal of 2b resulted in no consistent pattern of defects. Removal of 2c resulted in atypical shell development, absence of the heart, and eversión of the stomodeum; additional effects noted in some individuals involved the right velar lobe, the right statolith and perhaps the right eye. Anomalous birefringent bodies appeared frequently in the anterior region of the larva, on the right side after removal of 2c, and on the left side after removal of 2a. After removal of 2d the external shell was usually absent or rudimentary, the stomodeum was often everted, and other effects were noted in some individuals. On the basis of the deletion experiments, each second quartet micromere is judged to have a different embryonic value.     

Development of dorsoventral polarity and mesentoblast determination in Patella vulgata

In Patella vulgata the 32-cell stage represents a pause in the mitotic activity prior to the differentiation of the mesentoblast mother cell 3D. At the onset of this stage, the embryo is radially symmetrical. Nevertheless, the plane of bilateral symmetry is indicated as it passes through the macromeres forming the vegetal cross-furrow. From the early beginning of the 32-cell stage, all four macromeres intrude far into the interior and touch the centrally radiating cells of the first quartet of micromeres. The two cross-furrow forming macromeres (3B and 3D) intrude the farthest and come into contact with the greatest number of micromeres. Finally, the contacts are extended significantly and maintained with only one of these macromeres. From that moment, this cell can be called the macromere 3D and the dorsoventral axis is determined. The evolution of the internal cell contacts between the micromeres of the first quartet and the macromeres indicates an essential role of the former in the determination of one of the latter as the mesentoblast mother cell, and thus in the determination of dorsoventral polarity.     

Dorsoventral polarity and mesentoblast determination as concomitant results of cellular interactions in the mollusk Patella vulgata.

In mollusks with an equal four-cell stage, dorsoventral polarity becomes noticeable in the interval between the formation of the third and fourth quartet of micromeres, i.e., between the fifth and sixth cleavage. One of the two macromeres at the vegetal cross-furrow then partly withdraws from the surface and becomes located more toward the center of the embryonic cell mass than the other three macromeres. Only this specific macromere (3D) contacts the micromeres of the animal pole, divides with a delay, and develops into the stem cell of the mesentoblast (4d). After suppression of the normal contacts between micromeres and macromeres either by dissociation of the embryos or by deletion of first quartet cells, the normal differentiation of the macromeres fails to appear. By deleting a decreasing number of first quartet cells, an increasing percentage of embryos shows the normal differentiation pattern. Deletion of one of the cross-furrow macromeres does not preclude formation of the mesentoblast, which then originates by differentiation of an other macromere. It is concluded that initially the embryo is radially symmetrical and that the four quadrants have identical developmental capacities; mesentoblast differentiation from one macromere is induced through the contacts of the first quartet cells and that single macromere.     

Localization of morphogenetic determinants in a special cytoplasm present in the polar lobe ofBithynia tentaculata (Gastropoda)

Summary In the first polar lobe ofBithynia eggs a special plasm, the vegetal body, is present. It consists of a cupshaped aggregate of small vesicles. Centrifugation of eggs prior to first cleavage may result in displacement of the vegetal body. In about 50% of thecentriguged eggs the vegetal body is found outside the polar lobe, in one of the blastomeres. Removal of the polar lobe from non-centrifuged eggs always leads to severe defects in development. When the lobe is removed from centrifuged eggs, however, about 50% of the eggs develop into normal embryos. It is concluded that the presence of the vegetal body in a blastomere suffices to ensure normal development and, hence, that the polar lobe-specific morphogenetic determinants are contained within the vegetal body.     

Evolutionary modification of specification for the endomesoderm in the direct developing echinoid <Emphasis Type="Italic">Peronella japonica</Emphasis>: loss of the endomesoderm-inducing signal originating from micromeres

We investigated the inductive signals originating from the vegetal blastomeres of embryos of the sand dollar Peronella japonica, which is the only direct developing echinoid species that forms micromeres. To investigate the inductive signals, three different kinds of experimental embryos were produced: micromere-less embryos, in which all micromeres were removed at the 16-cell stage; chimeric embryos produced by an animal cap (eight mesomeres) recombined with a micromere quartet isolated from a 16-cell stage embryo; and chimeric embryos produced by an animal cap recombined with a macromere-derived layer, the veg1 or veg2 layer, isolated from a 64-cell stage embryo. Novel findings obtained from this study of the development of these embryos are as follows. Micromeres lack signals for endomesoderm specification, but are the origin of a signal establishing the oral–aboral (O–Ab) axis. Some non-micromere blastomeres, as well as micromeres, have the potential to form larval skeletons. Macromere descendants have endomesoderm-inducing potential. Based on these results, we propose the following scenario for the first step in the evolution of direct development in echinoids: micromeres lost the ability to send a signal endomesoderm induction so that the archenteron was formed autonomously by macromere descendants. The micromeres retained the ability to form larval spicules and to establish the O–Ab axis.     

High-resolution fate map of the snail Crepidula fornicata: the origins of ciliary bands, nervous system, and muscular elements

The littorinimorph gastropod Crepidula fornicata shows a spiralian cleavage pattern and has been the subject of studies in experimental embryology, cell lineage, and the organization of the larval nervous system. To investigate the contribution of early blastomeres to the veliger larva, we used intracellular cell lineage tracers in combination with high-resolution confocal imaging. This study corroborates many features derived from other spiralian fate maps (such as the origins of the hindgut and mesoderm from the 4d mesentoblast), but also yields new findings, particularly with respect to the origins of internal structures, such as the nervous system and musculature that have never been described in detail. The ectomesoderm in C. fornicata is mainly formed by micromeres of the 3rd quartet (principally 3a and 3b), which presumably represents a plesiomorphic condition for molluscs. The larval central nervous system is mainly formed by the micromeres of the 1st and 2nd quartet, of which 1a, 1c, and 1d form the anterior apical ganglion and nerve tracks to the foot and velum, and 2b and 2d form the visceral loop and the mantle cell. Our study shows that both first and second velar ciliary bands are generated by the same cells that form the prototroch in other spiralians and apparently bear no homology to the metatroch found in annelids.     

Localized binding of Dolichos lectin to the early Ascidia embryo

A fluorescent conjugate of Dolichos lectin has been used to investigate the surface of eggs and early embryos of Ascidia malaca. Unfertilized eggs show a patchy distribution of fluorescence. After fertilization, this pattern is retained until about the time of the emission of the second polar body, when the fluorescence becomes localized at the vegetal pole. This localization is retained in early development and by the 64-cell stage, binding is displayed by the two micromeres and the posterior vegetal macromeres.     

Development ofDentalium following removal of D-quadrant blastomeres at successive cleavage stages

Summary Each primary micromere and macromere of the D-quadrant ofDentalium was deleted, through the mesentoblast stage, to investigate the way in which the polar lobe cytoplasm exerts its influence on development.-D and -1D embryos form an apical tuft but no posttrochal structures.-2D embryos form an apical tuft and a reduced posttrochal region without a shell. -3D and -4D are externally similar to control embryos. -1d embryos and -1c embryos have an apical tuft with a reduced number of cilia. Embryos in which both 1c and 1d are deleted lack the apical tuft.-2d embryos lack shell and most other posstrochal structures. -3d and-4d embryos appear externally equivalent to controls.The polar lobe cytoplasm exerts its influence sequentially, and as inIlyanassa the maximal effect is at the third quartet stage.     

The embryonic value of the micromeres in Ilyanassa obsoleta,as determined by deletion experiments. III. The third quartet cells and the mesentoblast cell, 4d

Summary

Each of the third quartet micromeres and the mesentoblast of the fourth quartet was removed and the effects on larval development analyzed. Removal of 3a resulted in a reduction in size of the left velar lobe. Removal of 3b resulted in a moderate reduction in size of the right velar lobe. Removal of 3c resulted in the absence of the right half of the foot and usually the right statocyst. Removal of 3d resulted in the absence of the left half of the foot and the left statocyst. Removal of both 3c and 3d resulted in the absence of the foot in most cases. Removal of the mesentoblast, 4d, resulted in the absence of the intestine, heart and larval kidney and various deficiencies of the midgut. On the basis of deletion experiments, each third quartet micromere and the mesentoblast is judged to have a specific embryonic value. The generally good development of the main ectodermal derivatives of the body following removal of the mesentoblast do not suggest any role for it or its derivatives as the primary organizer of the body axis and form.     

The role of unequal cleavage and the polar lobe in the segregation of developmental potential during first cleavage in the embryo of Chætopterus variopedatus

Summary The inequality of the first cleavage division of the Chætopterus embryo is caused by the production of a small polar lobe and the internal shifting of the first cleavage spindle. This division produces a two-celled embryo containing a small AB and a large CD blastomere. These blastomeres have different morphogenetic potentials. Only the larvae resulting from isolated CD blastomeres are able to form bioluminescent photocytes, eyes and lateral hooked bristles. The removal of the polar lobe during first cleavage does not have a great effect on development. These lobeless embryos display a normal pattern of cleavages through the time of mesentoblast formation. The resulting larvae are essentially normal, however they do not form functional photocytes. If the CD cell is isolated after the removal of the first polar lobe, the resulting larva is virtually identical to those formed by the intact CD cell except it lacks the photocyte cells. These results indicate that two separate pathways are involved in the segregation of developmental or morphogenetic potential which takes place during first cleavage. One set of factors, which are necessary for photocyte formation, are associated with the first polar lobe. Other factors that are necessary for the formation of the eyes and lateral hooked bristles are segregated by the unequal cleavage which results from an internal shifting of the cleavage spindle. The removal of a large portion of the vegetal region of the embryo during first cleavage leads to the production of larvae which display a decreased ability to form eyes and lateral hooked bristles. These embryos frequently display an abnormal pattern of cleavages. They do not form the primary somatoblast or the mesentoblast. These results indicate that the vegetal region of the CD cell of Chætopterus is analogous to polar lobes which have been studied in other species, and is therefore important in the specification of the D quadrant. These features of the first cleavage of Chætopterus are a combination of those displayed by forms with direct unequal cleavage and other forms which cleave unequally through the production of large polar lobes. The significance of these findings is discussed relative to the origins of these different types of unequal cleavage.     

A Stereometric Analysis of Karyokinesis, Cytokinesis and Cell Arrangements during and following Fourth Cleavage Period in the Sea Urchin, Lytechinus variegatus

Fourth cleavage of the sea urchin embryo produces 16 blastomeres that are the starting point for analyses of cell lineages and bilateral symmetry. We used optical sectioning, scanning electron microscopy and analytical 3-D reconstructions to obtain stereo images of patterns of karyokinesis and cell arrangements between 4th and 6th cleavage. At 4th cleavage, 8 mesomeres result from a variant, oblique cleavage of the animal quartet with the mesomeres arranged in a staggered, offset pattern and not a planar ring. This oblique, non-radial cleavage pattern and polygonal packing of cells persists in the animal hemisphere throughout the cleavage period. Contrarily, at 4th cleavage, the 4 vegetal quartet nuclei migrate toward the vegetal pole during interphase; mitosis and cytokinesis are latitudinal and subequatorial. The 4 macromeres and 4 micromeres form before the animal quartet divides to produce a 12-cell stage. Subsequently, macromeres and their derivatives divide synchronously and radially through 8th cleavage according to the Sachs-Hertwig rule. At 5th cleavage, mesomeres and macromeres divide first; then the micromeres divide latitudinally and unequally to form the small and large micromeres. This temporal sequence produces 28-and 32-cell stages. At 6th cleavage, macromere and mesomere descendants divide synchronously before the 4 large micromeres divide parasynchronously to produce 56- and 60-cell stages.     

Morphological changes in the oocyte and its surrounding vestments during in vivo fertilization in the dasyurid marsupial Sminthopsis crassicaudata

This light and transmission electron microscopical study shows that the first polar body is given off before ovulation and that part of its cell membrane and that of the surrounding oocyte have long microvilli at the time of its ejection. Several layers of cumulus cells initially surround the secondary oocyte and first polar body, but the ovulated oocytes in the oviducts in the process of being fertilized do not have cumulus cells around them. Partly expelled second polar bodies occur in the oviduct; they are elongated structures that lack organelles and have electron-dense nuclei. A small fertilization cone appears to form around the sperm tail at the time of sperm entry into the egg and an incorporation cone develops around the sperm head in the egg cytoplasm. In three fertilized eggs a small hole was seen in the zona, which was presumably formed by the spermatozoon during penetration. Cortical granules, present in ovarian oocytes, are not seen in fertilized tubal or uterine eggs; release of their contents probably reduces the chances of polyspermy, although at least one polyspermic fertilized egg was seen and several other fertilized eggs had spermatozoa within the zona pellucida. In the zygote the pronuclei come to lie close together, but there was no evidence of fusion. A "yolk mass," which becomes eccentric before ovulation, is extruded by the time the two-cell embryos are formed, but many vacuoles remain in the non-yolky pole of the egg. A shell membrane of variable thickness is present around all uterine eggs but its origin remains undetermined.     

Polar effects of concanavalin A on the cortical cytoskeleton of a molluscan egg (Nassarius reticulatus,Gastropoda)

Summary The organization of the surface of fertilizedNassarius reticulatus eggs was probed by investigating the effects of treatment with concanavalin A (Con A). This lectin causes abnormal polar lobe formation as well as inhibition of cleavage. At low concentrations of Con A (0.3–1.0 μg/ml) the polar lobe constriction becomes considerably elongated, whereas at higher concentrations (2.5–50 μ/ml) the position of the constriction undergoes an extreme shift towards the animal pole. In the latter case, the surface of the animal part of the egg forms large protrusions and folds. Con A also causes resorption of microvilli and disappearance of the extracellular layer covering these villi; this process starts at the vegetal pole and propagates towards the animal pole. These changes in surface architecture are associated with profound changes in the organization of filamentous (F-) actin as assessed by confocal laser scanning microscopy of NBD-phallacidin-labelled eggs. Divalent succinyl-Con A has the same effects on polar lobe formation and surface architecture as tetravalent Con A, but only at very high concentrations (100–200 μg/ml), indicating that Con A exerts its effects by cross-linkage of its binding sites. Experiments with cytoskeleton inhibitors (cytochalasin D, colchicine, and nocodazole) reveal that in Con A-treated eggs — as in untreated eggs — microfilaments, but not microtubules, are involved in the formation of the polar lobe constriction. The calcium ion channel blocker D600 affects neither normal nor Con A-induced abnormal polar lobe formation, which suggests that influx of external calcium is not required. In contrast, treatment with TMB-8, an antagonist of internal calcium release, prevents the formation of a polar lobe in both normal and Con A-treated eggs. Finally, eggs from which the polar lobe has been removed prior to Con A treatment show none of the effects described, whereas isolated polar lobes react similarly to intact eggs. These results suggest that binding of Con A to sites present at the vegetal pole of the egg is responsible for the observed effects of the lectin.     

SPECIES SPECIFIC PATTERN OF CILIOGENESIS IN DEVELOPING SEA URCHIN EMBRYOS

The events of cell division and ciliogenesis in individual blastomeres of developing embryos of the sea urchins Temnopleurus toreumaticus and Hemicentrotus pulcherrimus were followed with a Nomarski differential interference microscope. The number of cell divisions before initiation of ciliogenesis was determined with respect to species. In T. toreumaticus , ciliogenesis began about 4 hr after fertilization at 25°C. The sequence of ciliogenesis was as follows: cilia appeared first on smaller micromeres, followed in order by blastomeres derived from larger micromeres, those from mesomeres and finally those derived from macromeres. Blastomeres originating from mesomeres, macromeres, larger micromeres and smaller micromeres had completed the 8th, 9th, 7th and 5th divisions respectively, before they generated cilia.
In H. pulcherrimus , embryos started to form cilia about 9 hr after fertilization at 18°C. Cilia appeared first on blastomeres of mesomere origin and, then on those of macromere origin. Before initiation of ciliogenesis, descendants of mesomeres and macromeres completed 9 and 10 rounds of cell division. Descendants of larger micromeres and the majority of cells derived from smaller micromeres did not acquired cilia even when the embryo began to rotate within the fertilization membrane. At this stage, the former had completed the 6th division and the latter the 8th division. Cell counts of blastomeres per embryo at the blastula stage also supported this observation.     

Clonal domains in postlarval Platynereis dumerilii (Annelida: Polychaeta)

Following an enzymatic procedure for softening the egg envelope, blastomeres in the embryo of the polychaete Platynereis dumerilii were injected with TRITC-dextran. Injection was successful in the following blastomeres: AB, CD, A, B, C, D, 1a-1d, 1A-1D, 4d, and 4d(1). The distribution of fluorescent label was recorded by confocal laser scanning microscopy of young, three-segmented worms after 3 or 4 days of development, in some cases also in 1-day-old trochophore larvae. Results were documented by single optical sections, by stacking a limited number or a complete set of optical sections, and by computer-generated surface views of both the labeled tissue domains and the body contours from complete image stacks of whole worms. With respect to their descent from the embryonic cell pattern, five major compartments can be distinguished which together compose the body of the young worm: 1) The epispheric, epidermal, and neural region of the head, composed of four domains arranged as quasi-radial sectors derived from micromeres 1a, 1b (left and right ventral), and 1c and 1d (right and left dorsal). 2) A posttrochal epidermal region of the head originating from micromeres 2a(1)-2c(1) and constituting the ventral and lateral posttrochal epidermis of the head. 3) A stomodeal-ectomesodermal region of the head, including the stomodeum (micromeres 2a(2) and 2c(2)), its mesodermal envelope, and head mesoderm (micromeres 3a-3d). 4) A solid cone composed of the four terminal macromeres 4A-4D, forming the core of the trunk as the endoderm anlage. 5) An epidermal and mesodermal coating of the trunk originating from the dorsal micromeres 2d and 4d. The region of the so-called (first, anterior) peristomial cirri at the posterior flanks of the head is also composed of 2d- and 4d-derived trunk tissue, thus corroborating the postulated descent of this region and its appendages from a cephalized anteriormost trunk segment and its parapodia. The cell-lineage domains of the first and third micromere tiers are arranged left or right of the sagittal plane, while two micromeres of the second quartet are in a lateral and, initially, two in a median position (2b ventral and 2d dorsal). The offspring of micromere 2d expand from a dorsal position toward the ventral midline and those of cell 4d from a posterior-dorsal site toward the anterior, initially forming two lateral bands. In the epispheric part of the head, part of the neurectodermal tissue derived from micromeres 1a and 1b interweaves in a medio-sagittal bar, and part of the first micromere offspring of all four quadrants (1a-1d) combine in forming a central brain neuropil. Each of the latter sends neurites through both of the circumesophageal connectives. Paired muscle tracts extend through the head toward the base of the antennae and are probably derived from micromeres 3a and 3b. A mesodermal envelope of the stomodeum is probably built by the 3c and 3d micromeres. The formation of symmetry and the nature of the body axes in the embryo and adult of Platynereis dumerilii are discussed. J. Morphol.