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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
_- : . .,
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
t
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
t
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
ti Wltb i
e Tunicate Fate p Ma
~ ~248
Js '. . F
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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