Ooplasmic segregation in theTubifex egg: Mode of pole plasm accumulation and possible involvement of microfilaments

Lophotrochozoan Zic Genes

Lophotrochozoa is a sister taxon of Ecdysozoa in the Protostomia that includes mollusks, annelids, brachiopods, and platyhelminths. Recent studies have clarified the structure, expression, and roles of lophotrochozoan Zic family genes. Zic genes in oligochaete annelid Tubifex tubifex (freshwater sludge worm) and polychaete annelid Capitella teleta (bristle worm) are commonly expressed in a subset of developing brain and mesoderm derivatives. The latter includes the naïve mesoderm and the associated chaetal sacs in each body segment, although the segmentation processes differ between the two species. Furthermore, in brachiopod Terebratalia transversa (lamp shell), Zic is expressed in the anterior ectodermal domains and mesodermal derivatives, including those associated with the chaetal sacs. This result suggests the common involvement of Zic genes in the development of chaetae, a lophotrochozoan novelty acquired in the course of evolution. In addition, the highly simplified lophotrochozoan Dicyema acuticephalum (dicyemid mesozoan, a cephalopod endoparasite), which lost its gut, nervous system, and muscles during evolution, expresses its Zic genes in hermaphroditic gonads, highlighting the role of Zic genes in germ cell development. The role of Zic in head regeneration was revealed in studies on platyhelminth Schmidtea mediterranea (freshwater planarian). Planarian Zic expression was induced in a subpopulation of neoblasts that includes adult pluripotent stem cells. It is needed for head regeneration and production of an anterior signaling center. Suppression of Wnt-β-catenin signaling underlies Zic-mediated head regeneration, reminiscent of Wnt-β-catenin suppression by vertebrate Zic genes. Taken together, studies on the lophotrochozoan Zic genes are essential to understanding not only the roles of these genes in body plan evolution but also the molecular mechanism underlying adult stem cell regulation.

Transcriptional control of unequal cleavage in early Tubifex embryos

Early embryos of the clitellate annelid Tubifex (oligochaete) undergo a series of unequal spiral cell divisions before the descendants of the D quadrant micromeres (cells 2d and 4d) divide bilaterally. Here, we show that inhibition of zygotic transcription by microinjection of α-amanitin (transcription inhibitor) exclusively converts unequal cleavage in cell 2d11 (granddaughter of 2d) into equal cleavage while other unequal cleavages and ensuing bilateral cleavages in cells 4d and 2d111 (great-granddaughter of 2d) all proceed in a normal fashion in the presence of this inhibitor. These results differ significantly from those reported for embryos of another clitellate annelid Helobdella (leech), in which inhibition of transcription converts bilateral (symmetric) cleavages in cells DNOPQ"' and DM" (equivalent to 2d111 and 4d) into unequal (asymmetric) cleavages while having no apparent effect on unequal cleavage in DNOPQ" (equivalent to 2d11). These differences imply distinct mechanisms for the control of the unequal-to-bilateral transition in the two clitellate annelids.

Developmental biology of the leech Helobdella

Glossiphoniid leeches of the genus Helobdella provide experimentally tractable models for studies in evolutionary developmental biology (Evo-Devo). Here, after a brief rationale, we will summarize our current understanding of Helobdella development and highlight the near term prospects for future investigations, with respect to the issues of: D quadrant specification; the transition from spiral to bilaterally symmetric cleavage; segmentation, and the connections between segmental and non-segmental tissues; modifications of BMP signaling in dorsoventral patterning and the O-P equivalence group; germ line specification and genome rearrangements. The goal of this contribution is to serve as a summary of, and guide to, published work.

D quadrant specification in the leech Helobdella: actomyosin contractility controls the unequal cleavage of the CD blastomere

The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo, respectively. Here we show that stereotyped cellular dynamics, including the formation of an intercellular blastocoel, culminate in a morphological left-right asymmetry in the 2-cell embryo, which precedes cytokinesis and predicts the chirality of the second cleavage. In contrast to the unequal first cleavage, the unequal second cleavage does not result from down-regulation of one centrosome, nor from an asymmetry within the spindle itself. Instead, the unequal cleavage of the CD cell entails a symmetric mitotic apparatus moving and anisotropically growing rightward in an actomyosin-dependent process. Our data reveal that mechanisms controlling the establishment of the D quadrant differ fundamentally even among the monophyletic clitellate annelids. Thus, while the homologous spiral cleavage pattern is highly conserved in this clade, it has diverged significantly at the level of cell biological mechanisms. This combination of operational conservation and mechanistic divergence begins to explain how the spiral cleavage program has remained so refractory to change while, paradoxically, accommodating numerous modifications throughout evolution.

Factors specifying cell lineages in the leech

As in arthropods, several major organ systems in leeches, including body musculature, nervous system and nephridia, are organized into a fixed number of longitudinally iterated units called segments. Many cells, especially neurons, can be uniquely identified from segment to segment. Leech embryos comprise identified cells, which facilitates developmental analysis. So far as it is known, cell lineages in leech are largely determinate. Prior to first cleavage, cytoplasmic reorganization generates domains of yolk-deficient cytoplasm called teloplasm. In situ hybridization experiments suggest that teloplasm is enriched for polyadenylated RNAs. During the first three, unequal cell divisions, teloplasm is segregated to macromere D'; normally, this cell alone cleaves further to generate five bilateral pairs of embryonic stem cells, M, N, O/P and Q teloblasts. Centrifugation experiments have shown a causal link between inheritance of teloplasm and the cleavage pattern that generates teloblasts. Teloblasts undergo highly unequal divisions, generating a longitudinal array of segmental founder cells called m, n, o, p and q blast cells, from which the definitive segmental tissues arise via further stereotyped cell divisions. Microinjecting new-born teloblasts or their precursors with polyadenylic acid induces the formation of supernumerary teloblasts. This discovery permits further analyses of factors specifying the five cell lines generating segmental tissues of the leech.

Asymmetric cell divisions in the early embryo of the leech Helobdella robusta

The small glossiphoniid leech Helobdella robusta is among the best-studied representatives of the super-phylum Lophotrochozoa in terms of early development. The Helobdella embryo undergoes a modified version of spiral cleavage, characterized by stereotyped cell lineages comprising multiple examples of equal and unequal divisions, many of which are well-conserved with respect to those of other clitellate annelids, such as the oligochaete Tubifex. Here, we review the early development of Helobdella, focusing on the variety of unequal cell divisions. We then summarize an experimental analysis of the mechanisms underlying the unequal first cleavage in Helobdella, concluding that the unequal first cleavages in Helobdella and Tubifex proceed by different mechanisms. This result demonstrates the evolvability of the basic cell biological mechanisms underlying well-conserved developmental processes. Finally, we propose a model in which the unequal second cleavage in Helobdella may be regulated by the polarized distribution of PAR protein homologs, convergent with the unequal first cleavage of the nematode Caenorhabditis elegans (super-phylum Ecdysozoa).

Reorganization of cytoplasm in the zebrafish oocyte and egg during early steps of ooplasmic segregation

The aim of this work is to determine when and how ooplasmic segregation is initiated in the zebrafish egg. To this end, the organization of the ooplasm and vitelloplasm were examined in oocytes and eggs shortly after activation. Ooplasmic segregation, initiated in the stage V oocyte, led to the formation of ooplasmic domains rich in organelles, and ribonucleoproteins. A linear array of closely arranged peripheral yolk globules separated an outer domain of ectoplasm from an inner domain of interconnected endoplasmic lacunae. The structure of this yolk array and the distribution of microinjected labeled tracers suggests that it may provide a barrier limiting ooplasm transit. Loosely arranged yolk globules at the animal hemisphere allow wide connections between the endoplasm and a preblastodisc domain. Activation caused further segregation of ooplasm, reorganization of endoplasmic lacunae, and blastodisc growth. The presence of an endoplasmic cytoskeleton suggests that these changes may be driven by microtubules and microfilaments.

Asymmetrization of first cleavage by transient disassembly of one spindle pole aster in the leech Helobdella robusta

Unequal first cleavage is characteristic of a diverse group of protostome animals. In the nematode Caenorhabditis elegans, unequal first cleavage is achieved through the interaction of an apparently symmetric mitotic spindle apparatus with a clearly polarized cell cortex. In the clitellate annelid Tubifex tubifex, by contrast, the spindle is monastral and contains only one gamma-tubulin-reactive centrosome; this monastral spindle is inherently asymmetric throughout mitosis. Here, we have used immunostaining for beta- and gamma-tubulin to follow spindle dynamics during the unequal first cleavage in another clitellate annelid, the leech Helobdella robusta. We find that the mitotic spindle is diastral and symmetric through early metaphase, then becomes asymmetric following the transient down-regulation of one centrosome, as judged by gamma-tubulin immunofluorescence. Low levels of drugs that affect microtubule dynamics can symmetrize the first cleavage without affecting the gamma-tubulin dynamics. Our results provide a striking example of the evolvability of cellular mechanisms underlying an unambiguously homologous developmental process.

Reorganization and translocation of the ectoplasmic cytoskeleton in the leech zygote by condensation of cytasters and interactions of dynamic microtubules and actin filaments

The formation and bipolar translocation of an ectoplasmic cytoskeleton of rings and meridional bands was studied in interphase zygotes of the glossiphoniid leech Theromyzon trizonare. Zygotes consisted of a peripheral organelle-rich ectoplasm and an internal yolk-rich endoplasm. After microinjection of labeled tubulin and/or actin, zygotes were examined by time-lapse video imaging, immunofluorescence and confocal microscopy. The rings and meridional bands were formed by condensation of a network of moving cytasters that represented ectoplasmic secondary centers of microtubule and actin filament nucleation. In some cases the network of cytasters persisted between the rings. The cytoskeleton had an outer actin layer and an inner microtubule layer that merged at the irregularly-shaped boundary zone. Bipolar translocation of the rings, meridional bands, or the network of cytasters led to accumulation of the cytoskeleton at both zygote poles. Translocation of the cytoskeleton was slowed or arrested by microinjected taxol or phalloidin, in a dose-dependent fashion. Results of drug treatment probably indicate differences in the degree and speed at which the cytoskeleton becomes stabilized. Moreover, drugs that selectively stabilized either microtubules or actin filaments stabilized and impaired movement of the entire cytoskeleton. Microtubule poisons and latrunculin-B failed to disrupt the cytoskeleton. It is concluded that the microtubule and actin cytoskeletons are dynamic, presumably cross-linked and resistant to depolymerizing drugs. They probably move along each other by a sliding mechanism that depends on the instability of microtubules and actin filaments.

Studies on fertilization in the teleost. I. Dynamic responses of fertilized medaka eggs

In order to understand the mechanisms of fertilization in the teleost, the movements of the egg cortex, cytoplasmic inclusions and pronuclei were observed in detail in fertilized medaka Oryzias latipes eggs. The first cortical contraction occurred toward the animal pole region following the onset of exocytosis of cortical alveoli. The cortical contraction caused movement of oil droplets toward the animal pole where the germinal vesicle had broken down during oocyte maturation. The movement of oil droplets toward the animal pole region was frequently twisted in the right or left direction. The direction of the twisting movement has been correlated with the unilateral bending of non-attaching filaments on the chorion. The female pronucleus, which approached the male pronucleus from the vicinity of the second polar body, took a course to the right, left or straight along the s-p axis connecting the male pronucleus and the second polar body. The course of approach by the female pronucleus correlated with the bending direction of the non-attaching filaments that had been determined by rotation of the oocyte around the animal-vegetal axis during oogenesis. The first cleavage furrow also very frequently coincided with the axis. These observations suggest that dynamic responses of medaka eggs from fertilization to the first cleavage reflect the architecture dynamically constructed during oogenesis.

Cell lineage in molluscan development

Cell lineage specification in molluscs is brought about by two mechanism: the segregation of morphogenetic plasms and inductive cell interactions. The evidence for the existence of morphogenetic plasms is largely circumstantial, but in one species, Bithynia, such a plasm has been identified in the polar lobe that forms at first cleavage. Inductive cell interactions are thought to be a prerequisite for the development of a large number of tissues and organs. The most extensively studied example is the specification of the mesodermal stem cell in Lymnaea and Patella, which occurs between 5th and 6th cleavage through an interaction between one macromere and a large number of micromeres. Both segregation and induction are tuned to the animal-vegetal polarity of the egg, at least during early development. This polarity probably arises during oogenesis and is manifest in regional differentiations of the surface architecture of the egg, in the distribution of inner membrane particles in the plasma membrane, in membrane fluidity characteristics, in ionic conductance properties of the plasma membrane, etc. All these phenomena have in common that they represent properties of the egg surface, suggesting that the polarity of the egg is somehow imprinted into the plasma membrane and the cortex of the egg during oogenesis.

An investigation of the specification of unequal cleavages in leech embryos

Leech embryos develop via stereotyped cell divisions, many of which are unequal. The first division generates identifiable cells, blastomeres AB and CD, which normally follow distinct developmental pathways. When these two cells are dissociated and cultured in isolation, their fates remain distinct and are reminiscent of normal development, but their typical cleavage patterns are disrupted; cell AB undergoes relatively few cell divisions, giving rise to a variable number of macromeres and micromeres, while cell CD cleaves many times, usually forming a poorly organized set of macromeres, embryonic stem cells (teloblasts), and micromeres. We have investigated the hypothesis that the abnormal cleavage pattern of isolated CD blastomeres is due to removal of mechanical constraints normally imposed by cell AB. We find that when cell CD is constrained in vitro to mimic its in vivo shape, it cleaves more normally.

Teloplasm formation in a leech, Helobdella triserialis, is a microtubule-dependent process

Fertilized eggs of the leech Helobdella triserialis undergo a cytoplasmic reorganization which generates domains of nonyolky cytoplasm, called teloplasm, at the animal and vegetal poles. The segregation of teloplasm to one cell of the eight-cell embryo is responsible for a unique developmental fate of that cell, i.e., to give rise to segmental ectoderm and mesoderm. We have studied the cytoplasmic movements that generate teloplasm using time-lapse video microscopy; the formation and migration of rings of nonyolky cytoplasm were visualized using transmitted light, while the movements of mitochondria into these rings were monitored with epifluorescence after labeling embryos with rhodamine 123, a fluorescent mitochondrial dye. To examine the likelihood that cytoskeletal elements play a role in the mechanism of teloplasm formation in Helobdella, we examined the distribution of microtubules and microfilaments during the first cell cycle by indirect immunofluorescence and rhodamine-phalloidin labeling, respectively. The cortex of the early embryo contained a network of microtubules many of which were oriented parallel to the cell surface. As teloplasm formation ensued, microtubule networks became concentrated in the animal and the vegetal cortex relative to the equatorial cortex. More extensive microtubule arrays were found within the rings of teloplasm. Actin filaments appeared in the form of narrow rings in the cortex, but these varied apparently randomly from embryo to embryo in terms of number, size, and position. The role of microtubules and microfilaments in teloplasm formation was tested using depolymerizing agents. Teloplasm formation was blocked by microtubule inhibitors, but not by microfilament inhibitors. These results differ significantly from those obtained in embryos of the oligochaete Tubifex hattai, suggesting that the presumably homologous cytoplasmic reorganizations seen in these two annelids have different cytoskeletal dependencies.

Animal and vegetal teloplasms mix in the early embryo of the leech, Helobdella triserialis

In embryos of the glossiphoniid leech, Helobdella triserialis, as in many annelids, cytoplasmic reorganization prior to first cleavage generates distinct animal and vegetal domains of yolk-deficient cytoplasm, called teloplasm. Both domains are sequestered to the D' macromere, progenitor of the definitive segmental tissues, during the first three rounds of cell division. And it has been believed that during the fourth round of cell division, the obliquely equatorial cleavage of macromere D' cleanly segregates animal teloplasm into an ectodermal precursor, cell DNOPQ, and vegetal teloplasm into a mesodermal precursor, cell DM. But here we report a hitherto unobserved cytoplasmic rearrangement between the second and the fourth divisions that seems to mix the animal and vegetal domains of teloplasm. The newly observed rearrangement consists of the movement of vegetal teloplasm toward the animal pole of cell D' between the second and the fourth cell divisions. Animal and vegetal teloplasms form a single pool of teloplasm in cell D' which is then divided between DM and DNOPQ at the fourth division. The movement of teloplasm was inferred by examination of embryos fixed and sectioned between the second and the fourth rounds of cleavage and was confirmed in living embryos microinjected with rhodamine 123, a fluorescent mitochondrial stain.

Polarization of ooplasmic segregation and dorsal-ventral axis determination in ascidian embryos

During ooplasmic segregation in ascidians, the myoplasm moves from its original location in the periphery of the unfertilized egg to the vegetal pole of the zygote. The vegetal cap of myoplasm marks the future site of gastrulation and the dorsal side of the embryo. The purpose of this investigation was to determine the mechanism for polarizing the myoplasm during ooplasmic segregation. To test the possibility that the myoplasm moves toward the sperm and that the vegetal pole is the exclusive site of sperm entry, we examined fertilization in egg fragments of the ascidians Styela plicata and Ciona savignyi. Similar frequencies of fertilization were exhibited by various egg fragments, including animal and vegetal fragments or multiple fragments prepared from the same egg. These results indicate that sperm do not enter the egg exclusively at the vegetal pole. Experiments with egg fragments and constricted eggs, combined with chalk marking of the animal pole, demonstrated that after fertilization the myoplasm segregates parallel to the animal-vegetal axis, usually toward the vegetal end of the cell. Activation of primary oocytes with the Ca2+ ionophore A23187 caused the myoplasm to segregate independently of the animal-vegetal axis. This confirms previous experiments in which eggs aligned along a glass fiber coated with A23187 segregated their myoplasm toward the fiber (W.R. Jeffery, 1982, Science 216, 545-547) and suggests that the intrinsic cue for polarization is a release of sequestered Ca2+ at fertilization. Therefore, it appears that ooplasmic segregation and the dorsal-ventral axis are polarized by maternal factors distributed in a concentration gradient along the animal-vegetal axis of the ascidian egg.

Localization of actin networks during early development of Tubifex embryos

In precleavage zygotes of Tubifex, actin filaments segregate to the animal and vegetal poles forming the polar actin filament networks (AFNs). In this study, the fate of the polar AFNs during early development of Tubifex embryos has been followed using rhodamine-phalloidin as a specific stain for F-actin. During the first two cleavages, which are unequal and meridional, the polar AFNs are retained at the regions of cells corresponding to the poles of the precleavage zygote; thereby, they are segregated to the CD-cell at the 2-cell stage then to the D-cell at the 4-cell stage. As the mitotic apparatus forms in the D-cell, however, the vegetal polar AFN translocates toward the animal pole of the cell where the mitotic apparatus is located and unites with the animal polar AFN there. This redistribution of the AFNs is impaired by colchicine treatment, suggesting the involvement of microtubules. Thereafter, the unified AFN is found to be associated with nuclear regions of the macromeres of the D-cell line, and finally partitioned to the teloblast precursors 2d and 4d and an endodermal cell 4D. Cytochalasin B experiments indicate that the AFNs play a cytoskeletal role in generating and maintaining the spatial organization of the cytoplasm which gives rise to the intracellular localization of the cytoplasm and the mitotic apparatus orientations. The developmental and cellular significance of the AFNs is discussed in relation to the localization of developmental potential and the regulation of the mitotic apparatus organization in the Tubifex embryo.

The effect of local cortical microfilament disorganization on ooplasmic segregation in the loach (Misgurnus fossilis) egg

Injections of cytochalasin D (CD) or DNase I under the surface of fertilized loach egg result in local disorganization of microfilamentous cortex (MC) as revealed by transmission electron microscopy. This effect correlates with the loss of the cortex ability to contract in vitro. The disorganization of MC in the vegetal hemisphere of the egg does not affect the ooplasm segregation or blastodisk cleavage. Injection under the animal pole suppresses blastodisk formation and results in the autonomous separation of ooplasm in the central part of the egg. The experiments suggest that (1) autonomous separation of ooplasm from the yolk granules can proceed in the central part of the egg without the participation of MC; (2) normal segregation of ooplasm at the animal pole requires that the structures of microfilaments in the animal hemisphere (but not in the vegetal one) be preserved.

Centrifugation redistributes factors determining cleavage patterns in leech embryos

In the normal development of glossiphoniid leech embryos, cytoplasmic reorganization prior to the first cleavage generates visibly distinct domains of yolk-deficient cytoplasm, called teloplasm. During an ensuing series of stereotyped and unequal cell divisions, teloplasm is segregated primarily into cell CD of the two-cell stage and then into cell D of the four-cell and eight-cell stages. The subsequent fate of cell D is also unique in that it alone undergoes further cleavages which generate five bilateral pairs of embryonic stem cells, the mesodermal (M) and ectodermal (N, O/P, O/P, and Q) teloblasts. Here we report studies on the effects of centrifugation on cleavage pattern and protein composition of individual blastomeres of the leech Helobdella triserialis. Centrifugation partially stratifies the cytoplasm of each cell, generating a layer of clear cytoplasm in cell CD derived largely from teloplasm. After centrifuging embryos at the two-cell stage, clear cytoplasm present in cell CD and normally inherited by cell D is redistributed and can be inherited by both cells C and D at the second cleavage. The developmental fates of cells C and D in centrifuged embryos correlate with the amount of clear cytoplasm they receive. In particular, when clear cytoplasm has been distributed roughly equally between the two cells, both cell C and cell D undergo further cleavages resembling the pattern of divisions normally associated with cell D. Likewise, non-yolk-associated proteins, normally found in higher quantities in cell D than in cell C, appear evenly disbursed between the two cells under conditions which induce this fate change. These results are consistent with the idea that the fates of cells C and D are influenced by the distribution or cellular localization of cytoplasmic components.

Influence of cytochalasin B on oocyte maturation in Xenopus laevis

Cytochalasin B (CB) exerts an inhibiting effect on the formation, migration and anchoring in the cortex of the meiotic spindle in maturing Xenopus laevis oocytes. Regional sensitivity to CB (CB-sensitive zones) has been found in the oocytes which varies with reference to the stage of oocyte maturation at which CB is applied. Light and electron microscopy has shown that in these CB-sensitive zones the yolk and pigment granules, unlike the cortical ones, are displaced into the cytoplasm centripetally under the influence of CB.

The spatial distribution of maternal mRNA is determined by a cortical cytoskeletal domain in Chaetopterus eggs

Messenger RNA molecules are localized in the cortical region of eggs and unevenly segregated to the embryonic cells during early development of the annelid Chaetopterus. The egg cortex is enriched in two organelles, ectoplasmic spherules and associated structures, which are similar in appearance to nuage. The physical basis of cortical mRNA localization was examined in stratified eggs and in eggs extracted with the nonionic detergent Nonidet P-40 (NP-40). The cortical organelles were displaced to the most centrifugal zone of stratified eggs. In situ hybridization with poly(U) or cloned DNA probes showed that a large proportion of the poly(A)+RNA, histone mRNA, and actin mRNA molecules was also displaced to the centrifugal zone. Extraction with NP-40 revealed a detergent-insoluble cytoskeletal domain (CD) in the egg cortex which contained the remnants of ectoplasmic spherules and nuage embedded in a fibrous network. Although most of the total protein and RNA was extracted by NP-40, a large proportion of the poly(A)+RNA, histone mRNA, and actin mRNA molecules was retained in the CD. In situ hybridization of stratified eggs extracted with NP-40 indicated that the CD, with its associated organelles and mRNA molecules, is displaced to the centrifugal zone as a unit. The results suggest that the tenacious association of mRNA molecules with the cortical CD may be responsible for maternal mRNA localization during early development.

Dynamics of the actin microfilament system in the Tubifex egg during ooplasmic segregation

Following the second polar body formation (PBF), the Tubifex egg undergoes ooplasmic segregation consisting of two steps, i.e., centrifugal migration of membranous organelles forming a subcortical ooplasmic layer and then movements of these organelles along the egg surface. The present investigation was undertaken to examine the microfilament organization in eggs during these ooplasmic rearrangements. Microfilaments throughout the egg are identified as actin by their reversible heavy meromyosin binding. Before the second PBF, a distinct network of actin filaments is present in the endoplasmic region. It is disorganized during the second PBF; short actin filaments are caused to aggregate with membranous organelles. Following the second PBF, similar short filaments become localized in the subcortical layer but not in the underlying yolky region. However, it is not until 50-60 min after the second PBF that an elaborate actin network is established in the subcortical layer. The cortex contains a sheet-like lattice of actin filaments. It is thickest around the animal pole, and tapes toward the equator of the egg. At about 90 min after the second PBF, this polarized distribution of cortical filaments becomes more pronounced as the result of their movements. Chronologically, subcortical actin network formation and cortical reorganization correspond to the later portion of the first step and the earlier portion of the second step of ooplasmic segregation, respectively. These findings are discussed in terms of ooplasmic movements and rearrangements.