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Gastrulation produces an embryo with three germ layers that are positioned to influence one another through inductive tissue interactions. During the next phase of development, called organogenesis, organs and organ systems develop simultaneously and in coordination with each other. In the chordates (see Key Concept 32.1), an early process of organogenesis is neurulation, the initiation of the nervous system. We will examine neurulation in the amphibian embryo, but it occurs in a similar fashion in reptiles (including birds), and in mammals.
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The chordamesoderm forms the notochord and initiates neurulation.
Segmented blocks of cells, known as somites, produce cells that develop into the vertebrae, ribs, trunk muscles, and limbs.
Hox genes control differentiation on the anterior−posterior axis of vertebrates.
Genetic and environmental factors can lead to neural tube defects.