Memoirs: The Direct Development of a New Zealand Ophiuroid

1941 ◽  
Vol s2-82 (327) ◽  
pp. 377-440
Author(s):  
H. BARRACLOUGH FELL
Keyword(s):  
Bilateral Symmetry ◽  
Ventral Surface ◽  
Radial Symmetry ◽  
Early Ontogeny ◽  
Body Cavity ◽  
Cell Stage ◽  
Animal Pole ◽  
Early Stages ◽  
Vegetal Pole ◽  
Primitive Structure

1. The first cleavage may be either equal, or markedly unequal; when it is equal the next segmentation affects both blastomeres; when it is unequal the larger blastomere is believed to give rise to three cells, and the smaller remains undivided till the next cleavage. 2. At the eight-cell stage there are two quartets of blastomeres. The upper quartet, micromeres, occupy the animal pole. The lower quartet, macromeres, occupy the vegetal pole. 3. The blastula comprises micromeres and macromeres, and the blastocoel is small and becomes eccentric. No cilia are developed. 4. The gastrula is formed by the shallow imagination of the macromeres, accompanied by an extensive process of epiboly affecting the micromeres. More marked epiboly of cells on two sides of the blastomere produces in the early stages two crests which later disappear. These may indicate a trace of bilateral symmetry. Epiblast comes to lie on solid mes-hypoblast. The archenteron is transient, and gives rise to no structures. The blastopore occupies the position of the definitive mouth. 5. No larva ever forms, nor is there any vestige of a larval stage. 6. The solid gastrula is converted into the adult by assuming a radial symmetry directly, with no intermediate bilaterally symmetrical form, unless the two epibolic crests are regarded as vestiges of larval symmetry. 7. The podia appear as solid outgrowths, in which the hydrocoelic cavity develops by splitting. 8. The definitive enteron appears as a split extending upward from the ventral surface through the solid hypoblast. 9. The young ophiuroid leaves the egg before the appearance of the general body cavity, and moves about, but does not at first take food. 10. The general coelomic body cavity and the perihaemal cavity develop by splitting in a mass of mesenchyme derived from the outer layers of mes-hypoblast. 11. The formation of the skeletal system is delayed till the stage of between two and three arm-segments. 12. The development of the skeleton follows closely that described for Amphiura squamata. 13. The tooth is shown to originate independently of the torus angularis; its rudiments comprise nine symmetrically disposed spicules. 14. The terminal plate arises later than the radials, and has a distinctive ‘primitive structure’. 15. The spine is shown to have a different development to that of the tooth, and therefore would seem to have no connexion with the latter in phylogeny or ontogeny. 16. It is suggested that the aberrant early stages are to be correlated with the retarding effect of the yolk mass present in the egg during ontogeny. The aberrant features may have had a different origin in phylogeny. 17. It is suggested that the simultaneous appearance in ontogeny of homologous organs situated at equal radial distances from the centre is to be explained in terms of hormonic activity. 18. It is concluded that evolution has considerably affected the early ontogeny without leaving its mark on phylogeny. The adult thus conforms to its class, the young form does not.

Specification of endoderm and mesoderm in the sea urchin

Zygote ◽  
1999 ◽  
Vol 8 (S1) ◽  
pp. S41-S41 ◽  
Author(s):  
David R. McClay
Keyword(s):  
Sea Urchin ◽  
Wnt Pathway ◽  
Special Significance ◽  
Nuclear Entry ◽  
Cell Stage ◽  
Animal Pole ◽  
Secondary Axis ◽  
Sea Urchin Embryo ◽  
Vegetal Pole

It has long been recognized that micromeres have special significance in early specification events in the sea urchin embryo. Micromeres have the ability to induce a secondary axis if transferred to the animal pole at the 16-cell stage of sea urchin embryos (Hörstadius, 1939). Without micromeres an isolated animal hemisphere develops into an ectodermal ball called a dauer blastula. Addition of micromeres to an animal half rescues a normal pluteus larva, including endoderm (Hörstadius, 1939). Despite these well-known experiments, however, neither the molecular basis of that induction nor the endogenous inductive role of micromeres in development was known. In recent experiments we learned that if one eliminates micromeres from the vegetal pole at the 16-cell stage the resulting embryo makes no secondary mesenchyme. Earlier it had been found that β-catenin is crucial for specification events that lead to mesoderm and endoderm (Wikra-manayake et al., 1998; Emily-Fenouil et al., 1998; Logan et al., 1999). We noticed that at the 16-cell stage β-catenin enters the nuclei of micromeres, then enters the nuclei of macromeres at the 32-cell stage (Logan et al., 1999). Since nuclear entry of β-catenin is known to be important for its signalling function in the Wnt pathway, we asked whether β-catenin functions in the micromere induction pathway.

scholarly journals Oriented cell divisions and cellular morphogenesis in the zebrafish gastrula and neurula: a time-lapse analysis

Development ◽  
1998 ◽  
Vol 125 (6) ◽  
pp. 983-994 ◽  
Author(s):  
M.L. Concha ◽  
R.J. Adams
Keyword(s):  
Cell Division ◽  
Time Lapse ◽  
Ventral Surface ◽  
Neural Plate ◽  
Common Mechanism ◽  
Gradual Transition ◽  
Animal Pole ◽  
Dorsal Midline ◽  
Vegetal Pole ◽  
The Central Nervous System

We have taken advantage of the optical transparency of zebrafish embryos to investigate the patterns of cell division, movement and shape during early stages of development of the central nervous system. The surface-most epiblast cells of gastrula and neurula stage embryos were imaged and analysed using a computer-based, time-lapse acquisition system attached to a differential interference contrast (DIC) microscope. We find that the onset of gastrulation is accompanied by major changes in cell behaviour. Cells collect into a cohesive sheet, apparently losing independent motility and integrating their behaviour to move coherently over the yolk in a direction that is the result of two influences: towards the vegetal pole in the movements of epiboly and towards the dorsal midline in convergent movements that strengthen throughout gastrulation. Coincidentally, the plane of cell division becomes aligned to the surface plane of the embryo and oriented in the anterior-posterior (AP) direction. These behaviours begin at the blastoderm margin and propagate in a gradient towards the animal pole. Later in gastrulation, cells undergo increasingly mediolateral-directed elongation and autonomous convergence movements towards the dorsal midline leading to an enormous extension of the neural axis. Around the equator and along the dorsal midline of the gastrula, persistent AP orientation of divisions suggests that a common mechanism may be involved but that neither oriented cell movements nor shape can account for this alignment. When the neural plate begins to differentiate, there is a gradual transition in the direction of cell division from AP to the mediolateral circumference (ML). ML divisions occur in both the ventral epidermis and dorsal neural plate. In the neural plate, ML becomes the predominant orientation of division during neural keel and nerve rod stages and, from late neural keel stage, divisions are concentrated at the dorsal midline and generate bilateral progeny (C. Papan and J. A. Campos-Ortega (1994) Roux's Arch. Dev. Biol. 203, 178–186). Coincidentally, cells on the ventral surface also orient their divisions in the ML direction, cleaving perpendicular to the direction in which they are elongated. The ML alignment of epidermal divisions is well correlated with cell shape but ML divisions within the neuroepithelium appear to be better correlated with changes in tissue morphology associated with neurulation.

Microtubule arrays of the zebrafish yolk cell: organization and function during epiboly

Development ◽  
1994 ◽  
Vol 120 (9) ◽  
pp. 2443-2455 ◽  
Author(s):  
L. Solnica-Krezel ◽  
W. Driever
Keyword(s):  
Danio Rerio ◽  
Developmental Changes ◽  
Microtubule Network ◽  
Cell Stage ◽  
Yolk Syncytial Layer ◽  
Animal Pole ◽  
Yolk Cell ◽  
Vegetal Pole ◽  
Cell Organization ◽  
And Function

In zebrafish (Danio rerio), meroblastic cleavages generate an embryo in which blastomeres cover the animal pole of a large yolk cell. At the 500–1000 cell stage, the marginal blastomeres fuse with the yolk cell forming the yolk syncytial layer. During epiboly the blastoderm and the yolk syncytial layer spread toward the vegetal pole. We have studied developmental changes in organization and function during epiboly of two distinct microtubule arrays located in the cortical cytoplasm of the yolk cell. In the anuclear yolk cytoplasmic layer, an array of microtubules extends along the animal-vegetal axis to the vegetal pole. In the early blastula the yolk cytoplasmic layer microtubules appear to originate from the marginal blastomeres. Once formed, the yolk syncytial layer exhibits its own network of intercrossing mitotic or interphase microtubules. The microtubules of the yolk cytoplasmic layer emanate from the microtubule network of the syncytial layer. At the onset of epiboly, the external yolk syncytial layer narrows, the syncytial nuclei become tightly packed and the network of intercrossing microtubules surrounding them becomes denser. Soon after, there is a vegetal expansion of the blastoderm and of the yolk syncytial layer with its network of intercrossing microtubules. Concomitantly, the yolk cytoplasmic layer diminishes and its set of animal-vegetal microtubules becomes shorter. We investigated the involvement of microtubules in epiboly using the microtubule depolymerizing agent nocodazole and a stabilizing agent taxol. In embryos treated with nocodazole, microtubules were absent and epibolic movements of the yolk syncytial nuclei were blocked. In contrast, the vegetal expansion of the enveloping layer and deep cells was only partially inhibited. The process of endocytosis, proposed to play a major role in epiboly of the yolk syncytial layer (Betchaku, T. and Trinkaus, J. P. (1986) Am. Zool. 26, 193–199), was still observed in nocodazole-treated embryos. Treatment of embryos with taxol led to a delay in all epibolic movements. We propose that the yolk cell microtubules contribute either directly or indirectly to all epibolic movements. However, the epibolic movements of the yolk syncytial layer nuclei and of the blastoderm are not coupled, and only movements of the yolk syncytial nuclei are absolutely dependent on microtubules. We hypothesize that the microtubule network of the syncytial layer and the animal-vegetal set of the yolk cytoplasmic layer contribute differently to various aspects of epiboly. Models that address the mechanisms by which the two microtubule arrays might function during epiboly are discussed.

scholarly journals Embryology and early ontogeny of an anemonefish Amphiprion ocellaris

2007 ◽  
Vol 87 (4) ◽  
pp. 1025-1033 ◽  
Author(s):  
Inayah Yasir ◽  
Jian G. Qin
Keyword(s):  
Coral Reef ◽  
Embryonic Development ◽  
Digestive Tract ◽  
Coral Reef Fish ◽  
Ventral Surface ◽  
Early Ontogeny ◽  
Egg Activation ◽  
Pectoral Fins ◽  
Vegetal Pole ◽  
Blood Formation

The present study describes the embryonic development and early ontogeny of Amphiprion ocellaris from fertilization to post hatching. Anemonefish spontaneously spawned at 27–28°C. The newly laid eggs were orange in colour and elliptical in shape (1.8×0.8 mm). Melanin appeared as a black mass situated at the vegetal pole in mature eggs. This is rarely seen in eggs of other fish species. We documented developmental times at 27–28°C to egg activation (0.5 h), cleavage (4 h), blastula (11.5 h), gastrula (20 h), neurula (24.5 h), somite (28.5 h), turnover (72 h), blood formation (113 h) and internal ear and jaw formation (144 h). Hatching occurred 152 h after fertilization. On day 4, the eye buds were pigmented and melanophores formed on the ventral surface of the embryo. Internal ear and gill formation were completed on day 5 and coincided with movement of the opercula and pectoral fins. The mouth formed on day 6 and the digestive tract appeared on day 7. By day 10, the yolk was fully absorbed and a substantial amount of food was observed in the gut. Dark and orange pigments were dispersed and aggregated through muscle contractions by day 14, but red pigments did not appear until the fish were three months old. This study contributes to a further understanding of the embryology and the early ontogeny of damselfish and may help improve the culture of coral reef fish.

Vegetal egg cytoplasm promotes gastrulation and is responsible for specification of vegetal blastomeres in embryos of the ascidian Halocynthia roretzi

Development ◽  
1996 ◽  
Vol 122 (4) ◽  
pp. 1271-1279 ◽  
Author(s):  
H. Nishida
Keyword(s):  
Radial Symmetry ◽  
Second Phase ◽  
Equatorial Position ◽  
Animal Cells ◽  
Halocynthia Roretzi ◽  
Animal Pole ◽  
Vegetal Pole ◽  
Ooplasmic Segregation ◽  
Derivatives Of ◽  
Egg Cortex

An animal-vegetal axis exists in the unfertilized eggs of the ascidian Halocynthia roretzi. The first phase of ooplasmic segregation brings the egg cortex to the vegetal pole very soon after fertilization. In the present study, when 5–8% of the egg cytoplasm in the vegetal pole region was removed between the first and second phase of segregation, most embryos exhibited failure of gastrulation, as reported previously in Styela by Bates and Jeffery (Dev. Biol, 124, 65–76, 1987). The embryos that were deficient in vegetal pole cytoplasm (VC-deficient embryos) developed into permanent blastulae. They consisted for the most part of epidermal cells and most lacked the derivatives of vegetal blastomeres, such as endoderm, muscle and notochord. Removal of cytoplasm from other regions did not affect embryogenesis. The cleavage of the VC-deficient embryos not only exhibited radial symmetry along the animal-vegetal axis but the pattern of the cleavage was also identical in the animal and vegetal hemispheres. Examination of the developmental fates of early blastomeres of VC-deficient embryos revealed that the vegetal blastomeres had assumed the fate of animal cells. These results suggested that the VC-deficient embryos had been totally animalized. When vegetal pole cytoplasm was transplanted to the animal pole or equatorial position of VC-deficient eggs, gastrulation occurred, starting at the site of the transplantation and tissues derived from vegetal blastomeres formed. Therefore, it appears that vegetal pole cytoplasm specifies the site of gastrulation and the cytoplasm is responsible for the specification of vegetal blastomeres. It is suggested that during the second phase of ooplasmic segregation, cytoplasmic factors responsible for gastrulation spread throughout the entire vegetal hemisphere.

Timing of the phases of the cell cycle during the period of asynchronous division up to the 49-cell stage in Lymnaea

Development ◽  
1971 ◽  
Vol 26 (3) ◽  
pp. 367-391
Author(s):  
J. A. M. van den Biggelaar
Keyword(s):  
Cell Cycle ◽  
Dna Synthesis ◽  
Bilateral Symmetry ◽  
Supporting Evidence ◽  
Cell Stage ◽  
Vegetative Cells ◽  
Animal Pole ◽  
Cell Cycles ◽  
Cell Embryo ◽  
In Cells

The duration of the phases of the cell cycle (M-G1–S-G2) has been determined from the 8-up to the 49-cell stage in eggs of Lymnaea, using autoradiography and cytophotometry of Feulgen-stained nuclei. Division asynchrony of corresponding cells in different quadrants is primarily caused by unequal lengthening of the G2 phases. In general it appeared that in the vegetative cells lengthening of the cell cycles is chiefly due to an extension of the G2 phases, whereas in the cells of the animal half the duration of both the S and the G2 phases are extended. DNA synthesis is not blocked in cells which stop dividing and start to differentiate. A conspicuous lengthening of the cell cycles is observed in the 16- and 24-cell embryo; this is accompanied with the reappearance of distinct nucleoli. Supporting evidence has been obtained for the assumption that bilateral symmetry at the animal pole of the embryo is induced by cells from the vegetative hemisphere, presumably by the macromere 3D, during the 24-cell stage.

scholarly journals Establishment of the axis in chordates: facts and speculations

Development ◽  
1997 ◽  
Vol 124 (12) ◽  
pp. 2285-2296 ◽  
Author(s):  
H. Eyal-Giladi
Keyword(s):  
Bilateral Symmetry ◽  
Radial Symmetry ◽  
External Signal ◽  
Germ Layer ◽  
Master Plan ◽  
Spemann's Organizer ◽  
Sperm Entry ◽  
Vegetal Pole ◽  
Maternal Determinants ◽  
Translocation Speed

A master plan for the early development of all chordates is proposed. The radial symmetry of the chordate ovum is changed at or after fertilization into a bilateral symmetry by an external signal. Until now two alternative triggers, sperm entry and gravity, have been demonstrated. It is suggested that a correlation exists between the amount of yolk stored in the egg and the mechanism used for axialization. The speed at which axialization of the embryo proper takes place depends on the translocation speed of maternal determinants from the vegetal pole towards the future dorsoposterior side of the embryo. On arrival at their destination, the activated determinants form, in all chordates, an induction center homologous to the amphibian ‘Nieuwkoop center’, which induces the formation of ‘Spemann's organizer’. On the basis of the above general scenario, a revision is proposed of the staging of some embryonic types, as well as of the identification of germ layer and the spaces between them.

scholarly journals Croonian Lecture: Evolution and symmetry in the order of the sea-pens

1918 ◽  
Vol 90 (626) ◽  
pp. 108-135 ◽  
Keyword(s):  
Posterior Extremity ◽  
Vertical Plane ◽  
Bilateral Symmetry ◽  
Ventral Surface ◽  
Radial Symmetry ◽  
Conclusive Evidence ◽  
The Body ◽  
The Other ◽  
Animal Kingdom ◽  
Proper Group

1. On Radial and Bilateral Symmetry in the Animal Kingdom . In a general survey of the animal kingdom two kinds of body symmetry are found—the bilateral and the radial. In many cases, genera, families, and even classes of animals show some structural departures from the completely radial or bilateral symmetry; in others, there is a combination of the two symmetries, as when an outer body form of radial symmetry covers bilater­ally symmetrical organs, and, further, there is now conclusive evidence that in the course of the evolution of certain groups of animals one form of body symmetry has been supplanted by the other. With all the varieties of form and structure adapted to the different conditions of life, and with all the complexities of development and organisation due to phylogeny, there are but few examples of animals that are completely bilateral or completely radial as regards all their organs, but the dominance of one or other of these symmetries is, in most cases, so far pronounced that any animal can be placed in its proper group on this method of classification. It is not possible to give in a few words a comprehensive definition of what is meant by the two symmetries; but if attention is confined for the moment to such types as the earthworm or a fish on the one hand, and to a polyp or a jelly-fish on the other hand, a basis for a definition may be found for each of the two symmetries. Thus, a bilaterally symmetrical animal is one in which the principal organs and appendages of the body are arranged in pairs on either side of a median vertical plane. Such an animal exhibits an anterior and a posterior extremity, a dorsal and a ventral surface, and a right and left side. And a radially symmetrical animal is one in which the principal organs and appendages of the body are arranged symmetrically on radial lines or planes proceeding from a common centre or a common axis. Radially symmetrical animals may be spherical or oval, dome- or disc-shaped, or cylindrical in form, and they do not exhibit anterior and posterior extremi­ties, dorsal and ventral surfaces, nor right and left sides.

scholarly journals Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances

2014 ◽  
Vol 369 (1648) ◽  
pp. 20130348 ◽  
Author(s):  
Lena C. Hileman
Keyword(s):  
Bilateral Symmetry ◽  
Radial Symmetry ◽  
Plant Evolution ◽  
Flowering Plant ◽  
Developmental Studies ◽  
Developmental Genetic ◽  
Evolutionary Transitions ◽  
Flower Symmetry ◽  
Molecular Phylogenetic ◽  
Developmental Programme

A striking aspect of flowering plant (angiosperm) diversity is variation in flower symmetry. From an ancestral form of radial symmetry (polysymmetry, actinomorphy), multiple evolutionary transitions have contributed to instances of non-radial forms, including bilateral symmetry (monosymmetry, zygomorphy) and asymmetry. Advances in flowering plant molecular phylogenetic research and studies of character evolution as well as detailed flower developmental genetic studies in a few model species (e.g. Antirrhinum majus , snapdragon) have provided a foundation for deep insights into flower symmetry evolution. From phylogenetic studies, we have a better understanding of where during flowering plant diversification transitions from radial to bilateral flower symmetry (and back to radial symmetry) have occurred. From developmental studies, we know that a genetic programme largely dependent on the functional action of the CYCLOIDEA gene is necessary for differentiation along the snapdragon dorsoventral flower axis. Bringing these two lines of inquiry together has provided surprising insights into both the parallel recruitment of a CYC -dependent developmental programme during independent transitions to bilateral flower symmetry, and the modifications to this programme in transitions back to radial flower symmetry, during flowering plant evolution.

The Effects of β-Mercaptoethanol on the Morphogenetic Movements of Amphibian Embryos

Development ◽  
1963 ◽  
Vol 11 (1) ◽  
pp. 155-166
Author(s):  
P. Malpoix ◽  
J. Quertier ◽  
J. Brachet
Keyword(s):  
Neural Tube ◽  
Tube Formation ◽  
Animal Pole ◽  
Morphogenetic Movements ◽  
Complete Development ◽  
Early Stages ◽  
Amphibian Embryos ◽  
Biological Specificity ◽  
Neural Tube Formation

The inhibition by β-mercaptoethanol of morphogenesis in amphibians, freshwater hydra, planarians and regenerating tadpoles, has already been reported by one of us (Brachet, 1958, 1959a, b, c). The present work provides a closer analysis of the biological specificity of j8-mercaptoethanol with regard to the different movements which produce gastrulation in amphibians: invagination, epiboly, convergent stretching and ingression. The main result, obtained with Pleurodeles, was that gastrulation is completely inhibited by M/100 β-mercaptoethanol. Lower concentrations (M/300) permit more complete development, but the resulting embryos are abnormal. β-Mercaptoethanol interferes with neural tube formation, but has less effect on the development of the notochord and the mesodermal somites. It was further noted that, when embryos are treated at very early stages (1–2 cells, young blastulae), the blastocoele seems to collapse and the ectoblast of the animal pole is deeply puckered.

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