Outline

1.1 Metazoans Evolved the Ability to Produce Cells with Very Different Functions

1.2 Preformationism Offered an Easy but Wrong Solution, While Epigenesis Seemed Incomprehensible

1.3 The Rediscovery of Genes Set the Stage for Understanding Development

1.4 Gene Expression Directs Cell Differentiation

• RESEARCHERS AT WORK Do Differentiating Cells Dispose of Unused Genes?

1.5 Scientists Domesticated a Simple Worm to Address the Questions of Cell Differentiation

1.6 Mitotic Lineage Guides Cell Differentiation in Worms

• BOX 1.1 KERFUFFLES IN LANGUAGE: “CELL FATE” AND “COMMITMENT”

1.7 Embryonic Development Begins by Forming Three Distinct Germ Layers

1.8 The Vertebrate Nervous System Begins as a Simple Tube

1.9 Many Embryos, Including All Vertebrates, Display “Self-Regulation”

1.10 Self-Regulation Seems Incompatible with Mitotic Lineage–Directed Differentiation

1.11 Experimental Embryology Revealed Inductive Processes Underlying Self-Regulation

1.12 A Region of the Vertebrate Embryo Seems to “Organize” Development

• RESEARCHERS AT WORK The Dorsal Lip of the Blastopore Can Organize a New Individual

1.13 Long Abandoned, the Organizer Was Uncovered through Molecular Biological Techniques

• RESEARCHERS AT WORK A Gene Is Discovered That Acts as an Organizer

1.14 What Organizes the Organizer?

1.15 In Insects, Epidermal Cells Compete to Become Neuroblasts