Activation of Egg Metabolism in Sea Urchins 139 The polar lobe: Cell determination and axis Release of intracellular calcium ions 139 formation 182 Effects of calcium 142 Gastrulation in Snails 185 m SiDELIGHTS SPECULATIONS Rules of Evidence: "Find It R SIDELIGHTS SPECULATIONS Adaptation by Modifying Lose It move It" 1 Embryonic Cleavage 186 Fusion of genetic material 14 EARLY DEVELOPMENT IN TUNICATES 187 Internal Fertilization in Mammals 145 Tunicate Cleavage 187 Getting the gametes into the oviduct: Translocation The tunicate fate map 187 Autonomous and conditional specification of In the vicinity of the oocyte: Hyperactivation tunicate blastomeres 188 thermotaxis, and chemotaxis 148 Specification of the embryonic axes 191 Recognition at the zona pellucida 149 Gastrulation in Tunicates 19 Gamete fusion and the prevention of THE NEMATODE C ELEGANS 192 polyspermy 152 Cleavage and Axis Formation in C elegans 193 Fusion of genetic material 153 H SIDELIGHTS SPECULATIONS The Nonequivalence of Rotational cleavage of the C elegans egg 193 Mammalian Pronuclei 154 Anterior-posterior axis formation 193 Activation of the mammalian egg 155 Formation of the dorsal-ventral and right-left axes 196 Coda 155 Control of blastomere identity 197 Gastrulation in C elegans 199 Coda 200 CHAPTER 5 Early Development in Selected chaPTeR 6 Invertebrates 159 The Genetics of Axis Specification EARLY DEVELOPMENTAL PROCESSES: AN in Drosophila 203 OVERVIEW 159 Cleavage 159 EARLY DROSOPHILA DEVELOPMENT 204 From fertilization to cleavage 160 Fertilization 204 The cytoskeletal mechanisms of mitosis 161 Cleavage 204 Patterns of embryonic cleavage 162 The mid-blastula transition 205 Gastrulation 162 Gastrulation 206 Cell Specification and Axis Formation 164 GENES THAT PATTERN THE DROSOPHILA EARLY DEVELOPMENT IN SEA URCHINS 165 BODY PLAN 208 Sea Urchin Cleavage 165 Primary Axis Formation during Oogenesis 209 Blastula formation 166 Anterior-posterior polarity in the oocyte 209 Fate maps and the determination of sea urchin Dorsal-ventral patterning in the oocyte 211 blastomeres 166 Generating the Dorsal-Ventral Pattern in the Global regulatory networks and skeletogenic Embryo 213 mesenchyme specification 16 Dorsal, the ventral morphogen 213 Specification of the vegetal cells 171 Establishing a nuclear Dorsal gradient 214 Axis specification 172 Effects of the Dorsal protein gradient 214 Sea Urchin Gastrulation 172 I SIDELIGHTS SPECULATIONS The Left-Right Axis 217 Ingression of the skeletogenic mesenchyme 172 Segmentation and the Anterior-Posterior Invagination of the archenteron 176 Body plan 218 EARLY DEVELOPMENT IN SNAILS 178 Maternal gradients: Polarity regulation by oocyte Cleavage in Snail Embryos 178 cytoplasm 219 The snail fate 181 The molecular model: Protein gradients in the early ryo 219Activation of Egg Metabolism in Sea Urchins 139 Release of intracellular calcium ions 139 Effects of calcium 142 • SIDELIGHTS & SPECULATIONS Rules of Evidence: "Find It, Lose It, Move It" 144 Fusion of genetic material 145 Internal Fertilization in Mammals 145 Getting the gametes into the oviduct: Translocation and capacitarion 145 In the vicinity of the oocyte: Hyperacrivation, thcrmotaxis, and chemo taxis 148 Recognition at the zona pellucida 149 Gamete fusion and the prevention of polyspermy 152 Fusion of genetic material 153 • SIDELIGHTS & SPECULATIONS The Nonequivalence of Mammalian Pronuclei 154 Activation of the mammalian egg 155 Coda 155 CHAPTER 5 Early Development in Selected Invertebrates 159 EARLY DEVELOPMENTAL PROCESSES: AN OVERVIEW 159 Cleavage 159 From fertilization to cleavage 160 The cytoskeletal mechanisms of mitosis 161 Patterns of embryonic cleavage 162 Gastrulation 162 Cell Specification and Axis Formation 164 EARLY DEVELOPMENT IN SEA URCHINS 165 Sea Urchin Cleavage 165 Blastula formation 166 Fate maps and the determination of sea urchin blastomeres 166 Global regulatory networks and skeletogenic mesenchyme specification 167 Specification of the vegetal cells 171 Axis specification 172 Sea Urchin Gastrulation 172 Ingression of the skeletogenic mesenchyme 172 Invagination of the archenteron 176 EARLY DEVELOPMENT IN SNAILS 178 Cleavage in Snail Embryos 178 The snail fate map 181 The polar lobe: Cell determination and axis formation 182 Gastrulation in Snails 185 • SIDELIGHTS & SPECULATIONS Adaptation by Modifying Embryonic Cleavage 186 EARLY DEVELOPMENT IN TUNICATES 187 Tunicate Cleavage 187 The tunicate fate map 187 Autonomous and conditional specification of tunicate blastomeres 188 Specification of the embryonic axes 191 Gastrulation in Tunicates 191 THE NEMATODE C. ELEGANS 192 Cleavage and Axis Formation in C. elegans 193 Rotational cleavage of the C. elegans egg 193 Anterior-posterior axis formation 193 Formation of the dorsal-ventral and right-left axes 196 Control of blastomere identity 197 Gastrulation in C. elegans 199 Coda 200 CHAPTER 6 The Genetics of Axis Specification in Drosophila 203 EARLY DROSOPHILA DEVELOPMENT 204 Fertilization 204 Cleavage 204 The mid-blastula transition 205 Gastrulation 206 GENES THAT PATTERN THE DROSOPHILA BODY PLAN 208 Primary Axis Formation during Oogenesis 209 Anterior-posterior polarity in the oocyte 209 Dorsal-ventral patterning in the oocyte 211 Generating the Dorsal-Ventral Pattern in the Embryo 213 Dorsal, the ventral morphogen 213 Establishing a nuclear Dorsal gradient 214 Effects of the Dorsal protein gradient 214 • SIDELIGHTS & SPECULATIONS The Left-Right Axis 217 Segmentation and the Anterior-Posterior Body Plan 218 Maternal gradients: Polarity regulation by oocyte cytoplasm 219 The molecular model: Protein gradients in the early embryo 219