X CONTENTS The anterior organizing center: The Bicoid and l SIDELIGHTS SPECULATIONS BMP4 and Geoffroy's Hunchback gradients 223 Lobster 267 The termimal gene group 225 The Regional Specificity of Neural Induction 267 Summarizing early anterior-posterior axis The head inducer: Wnt inhibitors 268 specification in Drosophila 226 Trunk patterning: Wnt signals and retinoic acid 270 Segmentation Genes 226 Specifying the Left-Right Axis 272 a SIDELIGHTS SPECULATIONS Segments and EARLY ZEBRAFISH DEVELOPMENT 273 Parasegments 227 Cleavage 275 The gap genes 228 The pair-rule genes 229 Gastrulation and Formation of the Germ Layers 277 The segment polarity genes 231 The Homeotic Selector Genes 234 Axis Formation in Zebrafish 278 Initiating and maintaining the patterns of homeotic Dorsal-ventral axis formation 278 gene expression 235 The fish Nieuwkoop center 281 Realisator genes 236 Anterior-posterior axis formation 282 Axes and Organ Primordia: The Cartesian ght axis formation 282 Coordinate Model 236 Coda 283 Coda 237 chaPTER 8 CHAPTER Z Birds and Mammals: Early Amphibians and Fish: Early Development and Axis Formation 287 Development and Axis Formation 241 EARLY DEVELOPMENT IN BIRDS 287 EARLY AMPHIBIAN DEVELOPMENT 242 Cleavage 287 Fertilization, Cortical Rotation, and Cleavage 242 Gastrulation of the Avian Embryo 288 Unequal radial holoblastic cleavage 24 The hypoblast 288 The mid-blastula transition: Preparing for The primitive streak 288 gastrulation 244 Molecular mechanisms of migration through the Amphibian Gastrulation 244 primitive streak 292 Vegetal rotation and the invagination of the bottle egression of the primitive streak and epiboly of the cells 245 ectoderm 293 I S/DEU/GHTS SPECULATIONS Fibronectin and the Axis Specification and the Avian "Organizer"295 Pathways for Mesodermal Migration 250 The role of gravity and the PMZ 295 Epiboly of the prospective ectoderm 251 The chick "organizer"296 Progressive Determination of the Amphibian Anterior-posterior patterning 297 Axes 252 Left-right axis formation 298 Hans Spemann: Inductive interactions in regulative EARLY MAMMALIAN DEVELOPMENT 300 ent 2 Cleavage 300 Hans Spemann and Hilde Mangold: Primary embrvonic induction 255 The unique nature of mammalian cleavage 300 Molecular Mechanisms of Amphibian Axis Compaction 301 Formation 256 a S/DEL/GHTS SPECULATIONS Trophoblast or ICM? 302 How does the organizer form? 257 Escape from the zona pellucida 304 Functions of the organ Mammalian Gastrulation 304 Induction of neural ectoderm and dorsal Modifications for development inside another BMP inhibitors 263 organism 305 cers: The BMPs 265 Formation of the extraembryonic membranes 307 N SIDELIGHTS SPECULATIONS Twins and Chimeras 309CONTENTS The anterior organizing center: The Bicoid and Hunchback gradients 223 The terminal gene group 225 Summarizing early anterior-posterior axis specification in Drosophila 226 Segmentation Genes 226 • SIDELIGHTS & SPECULATIONS Segments and Parasegments 227 The gap genes 228 The pair-rule genes 229 The segment polarity genes 231 The Homeotic Selector Genes 234 Initiating and maintaining the patterns of homeotic gene expression 235 Realisator genes 236 Axes and Organ Primordia: The Cartesian Coordinate Model 236 Coda 237 CHAPTER 7 Amphibians and Fish: Early Development and Axis Formation 241 EARLY AMPHIBIAN DEVELOPMENT 242 Fertilization, Cortical Rotation, and Cleavage 242 Unequal radial holoblastic cleavage 243 The mid-blastula transition: Preparing for gastrulation 244 Amphibian Gastrulation 244 Vegetal rotation and the invagination of the bottle cells 245 • SIDELIGHTS & SPECULATIONS Fibronectin and the : '-'r '.a\s for Mesodermal Migration 250 Epiboly of the prospective ectoderm 251 Progressive Determination of the Amphibian Axes 252 -:~f fre~2r-.: Inductive interactions in regulative development 253 Hans Spemann and Hilde Mangold: Primary embryonic induction 255 Molecular Vlechanisms of Amphibian Axis Formation 256 How does the organizer form? 257 Functions of the organizer 262 Induction of neural ectoderm and dorsal mesoderm: BMP inhibitors 263 Epidermal inducers: The BMPs 265 • SIDELIGHTS & SPECULATIONS BMP4 and Geoffrey's Lobster 267 The Regional Specificity of Neural Induction 267 The head inducer: Wnt inhibitors 268 Trunk patterning: Wnt signals and retinoic acid 270 Specifying the Left-Right Axis 272 EARLYZEBRAFISH DEVELOPMENT 273 Cleavage 275 Gastrulation and Formation of the Germ Layers 277 Axis Formation in Zebrafish 278 Dorsal-ventral axis formation 278 The fish Nieuwkoop center 281 Anterior-posterior axis formation 282 Left-right axis formation 282 Coda 283 CHAPTER 8 Birds and Mammals: Early Development and Axis Formation 287 EARLY DEVELOPMENT IN BIRDS 287 Cleavage 287 Gastrulation of the Avian Embryo 288 The hypoblast 288 The primitive streak 288 Molecular mechanisms of migration through the primitive streak 292 Regression of the primitive streak and epiboly of the ectoderm 293 Axis Specification and the Avian "Organizer" 295 The role of gravity and the PMZ 295 The chick "organizer" 296 Anterior-posterior patterning 297 Left-right axis formation 298 EARLY MAMMALIAN DEVELOPMENT 300 Cleavage 300 The unique nature of mammalian cleavage 300 Compaction 301 • SIDELIGHTS & SPECULATIONS Trophoblast or ICM? 302 Escape from the zona pellucida 304 Mammalian Gastrulation 304 Modifications for development inside another organism 305 Formation of the extraembryonic membranes 307 • SIDELIGHTS & SPECULATIONS Twins and Chimeras 309