DEVELOPMENTAL

BIOLOGY Sixth Editio n

SCOTT F. GILBERT

p a rr. Principles of development in biology

CHAPTER 1 Developmental biology:

Control of Developmental Morphogenesis: The Role of th eNucleus 3 1

The anatomicaltradition 3 Unicellular Protists and the Origins of Sexua l

The Questions of Developmental Biology 4

Reproduction 32

Anatomical Approaches to Developmental Biology 4

Multicellularity : The Evolution of Differentiation 3 5The Volvocaceans 3 5

Comparative Embryology 5Epigenesis and Preformation 6

Sidelights &Speculations: Sex and Individuality in

Naming the Parts : The Primary Germ Layers and EarlyVolvox 3 6

Organs 8

Differentiation and Morphogenesis in Dictyostelium : Cell

The Four Principles of Karl Ernst von Baer 9

Adhesion 39

Fate Mapping the Embryo 10

Sidelights & Speculations : Evidence and Antibodies 43

Cell Migration 13

Developmental Patterns among the Metazoa 4 4Evolutionary Embryology 14

Diploblasts 44

Embryonic Homologies 15

Protostomes and Deuterostomes 4 4

Medical Embryology and Teratology 17

Principles of Development : Life Cycles an dMathematical Modeling of Development 18

Developmental Patterns 4 7

The Mathematics of Organismal Growth 18

Literature Cited 4 7

The Mathematics of Patterning 1 9

Principles of Development : Developmental Anatomy 22

CHAPTER 3 Principles of experimenta lLiterature Cited 23

embryology 49

CHAPTER 2 Life cycles and the

Environmental Developmental Biology 49Environmental Sex Determination 50

evolution of developmental

Adaptation of Embryos and Larvae to Their Environments 5 0patterns 2S The Developmental Mechanics of Cell Specification 5 4

Autonomous Specification 54The Circle of Life : The Stages of Anima l

Development 25

Conditional Specification 56

The Frog Life Cycle 27

Syncytial Specification 6 6

The Evolution of Developmental Patterns inMorphogenesis and Cell Adhesion 66

Unicellular Protists 31

Differential Cell Affinity 67The Thermodynamic Model of Cell Interactions 69

Cadherins and Cell Adhesion 71

Silencers 122Principles of Development : Experimental

Locus Control Regions in Globin Genes 12 3Embryology 75

Methylation Pattern and the Control o fLiterature Cited 75

Transcription 125DNA Methylation and Gene Activity 12 5Possible Mechanisms by which Methylation Represses GeneCHAPTER 4 Genes and development : Transcription 125

Techniques and ethical issues 79 Sidelights & Speculations : Genomic Imprinting 12 6

The Embryological Origins of the Gene Theory 79

Transcriptional Regulation of An Entire Chromosome :Nucleus or Cytoplasm : Which Controls Heredity? 79

Dosage Compensation 12 6

The Split between Embryology and Genetics 81

Sidelights &Speculations : The Mechanisms of X

Early Attempts at Developmental Genetics 82

Chromosome Inactivation 128

Evidence for Genomic Equivalence 83

Differential RNA Processing 13 0Metaplasia 83

Control of Early Development by Nuclear RNA Selection 13 0Amphibian Cloning: The Restriction of Nuclear Potency 83

Creating Families of Proteins through Differential nRNA

Amphibian Cloning: The Pluripotency of Somatic Cells 85

Splicing 13 1

Cloning Mammals 87

Sidelights & Speculations : Differential nRNA

1Sidelights & Speculations : Why Clone Mammals? 88

Processing and Drosophila Sex Determination 13 3

Sidelights &Speculations : The Exception to the Rule :

Control of Gene Expression at the Level o f

Immunoglobin Genes 90 Translation 13 4Differential mRNA Longevity 13 4

Differential Gene Expression 91

Selective Inhibition of mRNA Translation 13 4RNA Localization Techniques 92

Control of RNA Expression by Cytoplasmic Localization 13 6Northern Blotting 92

Epilogue : Posttranslational Gene Regulation 13 6In Situ Hybridization 93

Principles of Development : Developmental Genetics 13 7Sidelights &Speculations : Whole-Mount In Situ

Literature Cited 13 8Hybridization 9 4

The Polymerase Chain Reaction 95

Determining the Function of Genes during

CHAPTER 6 Cell-cell communicationDevelopment 97

in development 143Transgenic Cells and Organisms 97

Induction and Competence 14 3Sidelights &Speculations: Human Germ Line Gene

Cascades of Induction : Reciprocal and Sequential InductiveTherapy 100

Events 145Determining the Function of a Message : Antisense RNA 102

Instructive and Permissive Interactions 14 6Identifying the Genes for Human Developmental

Epithelial-Mesenchymal Interactions 14 8Anomalies 103 Paracrine Factors 14 9

Principles of Development : Genes and

The Fibroblast Growth Factors 150--(- Development 105

Literature Cited 105

The Hedgehog Family 15 1The Wnt Family 152The TGF-/3 Superfamily 152

CHAPTER 5 The genetic core of

Other Paracrine Factors 153

development: Differential gene

Cell Surface Receptors and Their Signal Transductio n

expression 109 Pathways 15 3The RTK Pathway 154

Differential Gene Transcription 109

Sidelights & Speculations : The RTK Pathway and Cell -

Anatomy of the Gene : Exons and Introns 109

to-Cell Induction 156Anatomy of the Gene : Promoters and Enhancers 113

The Smad Pathway 158Transcription Factors 116

The JAK-STAT Pathway 159

Sidelights & Speculations : Studying DNA Regulatory

The Wnt Pathway 161

Elements 119

The Hedgehog Pathway 162

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Sidelights& Speculations : The Nature of Human

Direct Transmission of Signals through Gap Junctions 173

Syndromes 163

r Cross-Talk between Pathways 174

The Cell Death Pathways 165

Coda 175

xl: Juxtacrine Signaling 167

Principles of Development : Cell-Cel l

The Notch Pathway: Juxtaposed Ligands and Receptors 167

Communication 17 6

Sidelights & Speculations: Cell-Cell Interactions and

Literature Cited 17 7Chance in the Determination of Cell Types 169

€f e ~The Extracellular Matrix as a Source of Critica l

Developmental Signals 170

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CHAPTER 8 Early development ini

CHAPTER 7 Fertilization: Beginning a selected invertebrates 223 it- new organism 185

structure of the Gametes 185

An Introduction to Early DevelopmentalProcesses 223

Sperm 185

The Egg 188

Cleavage 22 3a

Recognition of Egg and Sperm 191

From Fertilization to Cleavage 22 4

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The Cytoskeletal Mechanisms of Mitosis 22 5Sperm Attraction: Action at a Distance 191

Patterns of Embryonic Cleavage 226The Acrosomal Reaction in Sea Urchins 193 Gastrulation 22 8Sidelights & Speculations : Action at a Distance:

Mammalian Gametes 194

Axis Formation 22 9

Species-Specific Recognition in Sea Urchins 196

The Early Development of Sea Urchins 22 9. . Gamete Binding and Recognition in Mammals 198

Cleavage in Sea Urchins 22 9Gamete Fusion and the Prevention of Polyspermy 199

Blastula Formation 230

z1! Fusion of the Egg and Sperm Plasma Membranes 199

Fate Maps and the Determination of Sea Urchin

1' ß ätThe Prevention of Polyspermy 200

Blastomeres 231

yF` rFisk .•

e Activation of Egg Metabolism 206

Sea Urchin Gastrulation 23 4Early Responses 206

Ingression of Primary Mesenchyme 23 5

Sidelights & Speculations : The Activation of Gamete

First Stage of Archenteron Invagination 23 8

Second and Third Stages of Archenteron Invagination 23 9Metabolism 208Late Responses 210

The Early Development of Snails 241

1

Fusion of the Genetic Material 210 tCleavage in Snail Eggs 241

I! 4 -Fusion of Genetic Material in Sea Urchins 210

, 1Fusion of Genetic Material in Mammals 212

Sidelights & Speculations: Adaptation by Modifying

fi gEmbryonic Cleavage 243

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Sidelights & Speculations : The Nonequivalence of

-44The Polar Lobe: Cell Determination and Axis Formation 244

p { ; tMammalian Pronuclei 213

Fate Map of Ilyanassa obsoleta 246 Ir f '" "

Rearrangement of the Egg Cytoplasm 214

J ,t VGastrulation in Snails 24 7Preparation for Cleavage 215

113Snapshot Summary 216

Early Development in Tunicates 24 7Literature Cited 217

Tunicate Cleavage 247

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Autonomous and Conditional Specification of Tunicate

Sidelights &Speculations : The HomeodomainBlastomeres 249

Proteins 28 9Specification of the Embryonic Axes 25 0

Gastrulation in Tunicates 251

The Generation of Dorsal-Ventral Polarity 29 0

Early Development of the Nematode

The Morphogenetic Agent for Dorsal-Ventral

Caenorhabditis elegans 251

Polarity 290

Why C. elegans? 251

The Translocation of Dorsal Protein 290

The Signal Cascade 29 1Cleavage and Axis Formation in C. elegans 253

Establishing the Dorsal Protein Gradient 294Rotational Cleavage of the C . elegans Egg 253

Axes and Organ Primordia : The Cartesian Coordinat eAnterior-Posterior Axis Formation 253

Model 296Formation of the Dorsal-Ventral and Right-Left Axes 253

Coda 297Control of Blastomere Identity 253Autonomous Specification 254

Snapshot Summary 29 7

Conditional Specification 255

Literature Cited 29 8

Gastrulation in C. elegans 257

Coda 257

CHAPTER 10 Early development andSnapshot Summary 257

axis formation in amphibians 303Literature Cited 258

Early Amphibian Development 303CHAPTER 9 The genetics of axis

Cleavage in Amphibians 303

specification inDrosophila 263 Amphibian Gastrulation 30 5The Fate Map of Xenopus 305

Early Drosophila Development 263

Cell Movements during Amphibian Gastrulation 30 6

The Midblastula Transition: Preparing for Gastrulation 308Cleavage 264

Positioning the Blastopore 308The Midblastula Transition 265

Invagination and Involution 30 9Gastrulation 266

The Convergent Extension of the Dorsal Mesoderm 31 1

The Origins of Anterior-Posterior Polarity 269

Migration of the Involuting Mesoderm 31 2

The Maternal Effect Genes 270

Epiboly of the Ectoderm 31 2

Embryological Evidence of Polarity Regulation by Oocyte

Sidelights & Speculations : Fibronectin and th e

Cytoplasm 270

Pathways for Mesodermal Migration 31 3

The Molecular Model : Protein Gradients in the EarlyEmbryo 270

Axis Formation In Amphibians: TheEvidence that the Bicoid Gradient Constitutes the Anterior

Phenomenon of the Organizer 31 4Organizing Center 271

The Progressive Determination of the Amphibian

The Posterior Organizing Center: Localizing and Activating

Axes 314

Nanos 276

Hans Spemann and Hilde Mangold: Primary

The Terminal Gene Group 277

Embryonic Induction 31 7

The Segmentation Genes 278

The Mechanisms of Axis Formation in Amphibian s

The Gap Genes 279

31 8

The Pair-Rule Genes 280

The Origin of the Nieuwkoop Center 31 9

The Segment Polarity Genes 282

The Molecular Biology of the Nieuwkoop Center 31 9

The Homeotic Selector Genes 285

The Functions of the Organizer 32 2

Patterns of Homeotic Gene Expression 285

The Diffusible Proteins of the Organizer I: The BMP

Initiating the Patterns of Homeotic Gene Expression 286

Inhibitors 324

Maintaining the Patterns of Homeotic Gene Expression 287

Sidelights & Speculations: BMP4 and Geoffroy 's

Realisator Genes 287

Lobster 327

The Diffusible Proteins of the Organizer II: The Wnt

Inhibitors 328

Conversion of the Ectoderm into Neural Plate Cells 329

The Primitive Streak 348

Sidelights & Speculations : Competence, Bias, and

Epiboly of the Ectoderm 35 0

Neurulation 330

Axis Formation in the Chick Embryo 35 0The Regional Specificity of Induction 331

The Role of pH in Forming the Dorsal-Ventral Axis 35 0

The Determination of Regional Differences 331

The Role of Gravity in Forming the Anterior-Posterior

Molecular Correlates of Neural Caudalization 331

Axis 351

Specifying the Left-Right Axis 333

Left Right Axis Formation 353

Snapshot Summary 334

Early Mammalian Development 354Literature Cited 335

Cleavage in Mammals 354x .- . .

The Unique Nature of Mammalian Cleavage 354

CHAPTER 11 The early development

Compaction 356

of

vertebrates : Fish, birds, and Escape from the Zona Pellucida 35 7

It II II

Gastrulation in Mammals 35 8Modifications for Development within Anothe r

Organism 358; 4 . : Earl Development in Fish 33 9y

Formation of Extraembryonic Membranes 361' x .

Cleavage in Fish Eggs 339

j Sidelights & Speculations : Twins 362Gastrulation in Fish Embryos 341

Mammalian Anterior-Posterior Axis Formation 36 4The Formation of Germ Layers 341

Two Signaling Centers 364Axis Formation in Fish Embryos 343

Patterning the Anterior-Posterior Axis : The Hox CodeDorsal-Ventral Axis Formation : The Embryonic Shield 343

Hypothesis 365Anterior-Posterior Axis Formation : Two Signaling

Expression of Hox Genes along the Dorsal Axis 36 7Centers 345

Experimental Analysis of the Hox Code 367Left-Right Axis Formation 345

The Dorsal-Ventral and Left-Right Axes i n

'Early Development In Birds 345

Mammals 36 9

leavage in Bird Eggs 345

The Dorsal-Ventral Axis 36 9

astrulation of the Avian Embryo 347

The Left-Right Axis 370

e Hypoblast 347Snapshot Summary 372Literature cited 373

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HAPTER 12 The central nervous

Adult Neural Stem Cells 394

system and the epidermis 379 Neuronal Types 39 5Sidelights & Speculations: Homologous Specification

Formation of the Neural Tube 379

of Neural Tissue between Vertebrates andPrimary Neurulation 380

Arthropods 398Secondary Neurulation 385

Development of the Vertebrate Eye 399ifferentiation of the Neural Tube 386

The Dynamics of Optic Development 400

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The Anterior-Posterior Axis 387

Neural Retina Differentiation 40 1The Dorsal-Ventral Axis 388

Lens and Cornea Differentiation 40 1

Tissue Architecture of the Central Nervous System 378

The Epidermis and the Origin of CutaneousSpinal Cord and Medulla Organization 390

Structures 403Cerebellar Organization 390

The Origin of Epidermal Cells 403

Cerebral Organization 392

Cutaneous Appendages 404

Patterning of Cutaneous Appendages 404

Muscle Cell Fusion 453Snapshot Summary 406

Osteogenesis : The Development of Bones 454Literature Cited 407

Intramembranous Ossification 455Endochondral Ossification 45 6

CHAPTER 13 Neural crest cells and

Sidelights &Speculations : Control of CartilageMaturation at the Growth Plate 458axonal specificity 411 Osteoclasts 459

Intermediate Mesoderm 46 0The Neural Crest 411

Progression of Kidney Types 460The Trunk Neural Crest 413

Reciprocal Interaction of Kidney Tissues 461Migration Pathways of Trunk Neural Crest Cells 413

The Mechanisms of Reciprocal Induction 46 1The Mechanisms of Trunk Neural Crest Migration 414

Snapshot Summary 46 6Trunk Neural Crest Cell Differentiation 415

Literature Cited 46 7The Cranial Neural Crest 41 7

Sidelights Q'? Speculations: Tooth Development 420

CHAPTER 15 Lateral plate mesodermThe Cardiac Neural Crest 422

and endoderm 471Neuronal Specification and Axona lSpecificity 422

Lateral Plate Mesoderm 47 1The Generation of Neuronal Diversity 423

The Heart 47 1Pattern Generation in the Nervous System 425

Specification of Heart Tissue and Fusion of Hear tCell Adhesion and Contact Guidance: Attractive and

Rudiments 472Permissive Molecules 426

Looping and Formation of Heart Chambers 47 5Guidance by Axon-Specific Migratory Cues : The Labeled

Sidelights & Speculations : Redirecting Blood Flow inPathways Hypothesis 426

the Newborn Mammal 47 7Contact Guidance by Specific Growth Cone Repulsion 42 7Guidance By Diffusible Molecules 428 Formation of Blood Vessels 48 0

Target Selection 431

Constraints on How Blood Vessels May Be Constructed 480

Address Selection : Activity-Dependent Development 433

Vasculogenesis: Formation of Blood Vessels from Bloo dIslands 48 1Differential Survival after Innervation : Neurotrophic

Angiogenesis: Sprouting of Blood Vessels and Remodeling ofFactors 434

Vascular Beds 483Paths to Glory: Migration of the Retinal Ganglion Axons 435The Development of Blood Cells 48 5

Sidelights & Speculations : Fetal Neurons in Adult

The Stem Cell Concept 485Hosts 438

Pluripotential Stem Cells and Hematopoieti cThe Development of Behaviors: Constancy and

Microenvironments 485Plasticity 439

Sites of Hematopoiesis 48 8Snapshot Summary 43 9

Literature Cited 440

Endoderm 490The Pharynx 490

CHAPTER 14 Paraxial and

The Digestive Tube and Its Derivatives 49 1

mesoderm 447 Specification of the Gut Tissue 492

intermediate Liver, Pancreas, and Gallbladder 493

Paraxial Mesoderm : The Somites and Their

Sidelights &Speculations: The Specification of Live r

Derivatives 447

and Pancreas 494

The Initiation of Somite Formation 448

The Respiratory Tube 49 5

Specification and Commitment of Somitic Cell Types 451

The Extraembryonic Membranes 49 5

Determining Somitic Cell Fates 452

The Amnion and Chorion 49 6

Myogenesis : The Development of Muscle 453

The Allantois and Yolk Sac 49 7Specification and Differentiation by the Myogenic bHLH

Snapshot Summary 49 7

Proteins 453

Literature Cited 498

CHAPTER 16 Development of the

Location-Dependent Sex Determination in Bonellia andCrepidula 54 1tetrapod limb 503 Snapshot Summary 542

Formation of the Limb Bud 504

Literature Cited 54 3Specification of the Limb Fields: Hox Genes and Retinoic

Acid 504

CHAPTER 18 Metamorphosis,Induction of the Early Limb Bud: Fibroblast Growth Factors

504

regeneration, and aging 547Specification ofForelimb or Hindlimb : Tbx4 and TbxS 505Induction of the Apical Ectodermal Ridge 506

Metamorphosis: The Hormonal ReactivationGenerating the Proximal-Distal Axis of the Limb 508

of Development 54 7The Apical Ectodermal Ridge: The Ectodermal

Amphibian Metamorphosis 54 8Component 508

Morphological Changes Associated with MetamorphosisThe Progress Zone : The Mesodermal Component 509

548

Hox Genes and the Specification of the Proximal-Distal

Biochemical Changes Associated with Metamorphosis 54 9Axis 511

Hormonal Control of Amphibian Metamorphosis 54 9Sidelights & Speculations : Hox Genes and the

Sidelights & Speculations : Raising Tadpoles 552Evolution of the Tetrapod Limb 513

Molecular Responses to Thyroid Hormones duringSpecification of the Anterior-Posterior Limb Axis 513

Metamorphosis 552The Zone ofPolarizing Activity 513

Sidelights &Speculations: Heterochrony 554Sonic Hedgehog Defines the ZPA 514

Metamorphosis In Insects 55 6The Generation of the Dorsal-Ventral Axis 515

Types of Insect Metamorphosis 556Coordination among the Three Axes 516

Eversion and Differentiation of the Imaginal Discs 55 7Cell Death and the Formation of Digits and Joints 517

Sidelights &Speculations: Determination of the Win gSculpting the Autopod 517

Imaginal Discs 559Forming the Joints 517

Hormonal Control of Insect Metamorphosis 562Snapshot Summary 519

The Molecular Biology ofHydroxyecdysone Activity 563Literature Cited 519

Regeneration S65Epimorphic Regeneration of Salamander Limbs 566

CHAPTF,R17Sex determination 523

Formation of the Apical Ectodermal Cap and Regeneratio nBlastema 566

Chromosomal Sex Determination in Mammals 524

Proliferation of the Blastema Cells: The Requirement fo rPrimary and Secondary Sex Determination 524

Nerves 568The Developing Gonads 525

Pattern Formation In the Regeneration Blastema 56 8The Mechanisms of Mammalian Primary Sex

Compensatory Regeneration in the Mammalian Live rDetermination 527

570Secondary Sex Determination : Hormonal Regulation of th e

Sexual Phenotype 531

Morphallactic Regeneration in Hydras 570

I Sidelights & Speculations: Sex Determination and

Sidelights & Speculations : Medical Advances in

Behaviors 534

Regeneration 57 3

Chromosomal Sex Determination in Drosophila 536

Aging: The Biology of Senescence 57 4The Sexual Development Pathway 536

Maximum Life Span and Life Expectancy 57 4The Sex-lethal Gene as the Pivot for Sex Determination 537

Causes of Aging 57 5The Transformer Genes 539

Oxidative Damage 575Doublesex: The Switch Gene ofSex Determination 540

General Wear-and-Tear and Genetic Instability 576Environmental Sex Determination 540

Snapshot Summary 577emperature-Dependent Sex Determination in Reptiles 540

terature Cited 578

CHAPTER 19 The saga of the germ

Meiosis 59 5

line S8S Sidelights & Speculations: Big Decisions : Mitosis orMeiosis? Sperm or Egg? 59 7

Germ Plasm and the Determination of the Primordial

Spermatogenesis 599Germ Cells 585

Oogenesis 602Germ Cell Determination in Nematodes 586

Oogenic Meiosis 602Germ Cell Determination in Insects 587

Maturation of the Oocyte in Amphibians 603Germ Cell Determination in Amphibians 588

Completion of Amphibian Meiosis: Progesterone an dGerm Cell Migration 589

Fertilization 605Germ Cell Migration in Amphibians 589

Gene Transcription in Oocytes 606Germ Cell Migration in Mammals 590

Meroistic Oogenesis in Insects 608

Sidelights & Speculations: EG Cells, ES Cells, and

Oogenesis in Mammals 61 0Teratocarcinomas S92

Snapshot Summary 61 3

Germ Cell Migration in Birds and Reptiles 593

Literature Cited 61 4Germ Cell Migration in Drosophila 593

p a r t_ Ramificationsof developmental biology

CHAPTER 20 An overview of plant

CHAPTER 21 Environmentaldevelopment 621 regulation of animalPlant Life Cycles 622

development 649Gamete Production in Angiosperms 62 4

Pollen 624

Environmental Regulation of Norma lThe Ovary 626

Development 64 9

Pollination 627

Environmental Cues and Normal Development 649

Fertilization 629

Larval Settlement 64 9

Embryonic Development 629

Blood Meals 650

Developmental Symbiosis 65 0Experimental Studies 62 9

Embryogenesis 630

Predictable Environmental Differences as Cues for

Dormancy 633

Development 65 2

Seasonality and Sex in Aphids 652Germination 634

Diapause 653Vegetative Growth 635

Gravity and Pressure 65 3Meristems 635

Phenotypic Plasticity : Polyphenism and Reactio nRoot Development 636

Norms 65 3Shoot Development 637

Seasonal Polyphenism in Butterflies 65 5Leaf Development 638

Nutritional Polyphenism 655The Vegetative-to-Reproductive Transition 639

Environment-Dependent Sex Determination 65 6

Senescence 645

Polyphenisms for Alternative Conditions 65 7

Snapshot Summary 645

Predator-Induced Defenses 658

Literature Cited 646

Mammalian Immunity as a Predator-Induced

Response 660

Sidelights & Speculations : Genetic Assimilation 661

Sidelights & Speculations: Assessing Homologie sLearning: An Environmentally Adaptive Nervous

through Regulatory Gene Expression Patterns 68 9

System 662

Homologous Pathways of Development 69 0Instructions for Forming the Central Nervous System 69 0

Environmental Disruption of Normal

Limb Formation 692Development 664Teratogenic Agents 665

Modularity: The Prerequisite for Evolution throug hDevelopment 69 3

Retinoic Acid as a Teratogen 666Dissociation : Heterochrony and Allometry 694

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Alcohol as a Teratogen 667 Duplication and Divergence 69 5Other Teratogenic Agents 668

Co-option 69 6Sidelights & Speculations : Endocrine Disruptors 669

Developmental Correlation 69 7Genetic-Environmental Interactions 673

Correlated Progression 69 7Coda 673

Coevolution of Ligand and Receptor 698Snapshot Summary 674

Developmental Constraints 69 9Literature Cited 674

Physical Constraints 69 9z

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Morphogenetic Constraints 700

CHAPTER 22 Developmental Phyletic Constraints 700

mechanisms of evolutionary

' Sidelights & Speculations: Canalization and theRelease of Developmental Constraints 70 2

change679 A New Evolutionary Synthesis 70 2

"Unity of Type" and "Conditions of Existence" 679

Snapshot Summary 705Charles Darwin's Synthesis 679

Literature Cited 706E. B. Wilson and F. R. Lillie 680

j Life's Splendid Drama" 680The Search for the Urbilaterian Ancestor 682

Appendix I: A Partial List of Genes Active in Huma n

`Hox Genes : Descent with Modification 683

Development 71 0

. Changes in Hox-Responsive Elements of Downstream Genes

Sources for Chapter-Opening Quotations 71 2684

hanges in Hox Gene Transcription Patterns within a Bodys Portion 684

Author Index 71 3hanges in Hox Gene Expression between Body Segments

686Changes in Hox Gene Number 688

Subject Index 725

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