CHAPTER 29 Development, Growth, Aging, and Genetics

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TABLE 29.1

Germ Layer Derivatives

Neural grooveNeural foldNotochord

Neural plate

1 The neural plate forms fromectoderm.

2 Neural folds form as parallelridges along the embryo.

3 Neural crest cells begin to formfrom the crest of the neural folds.

4

The neural folds meet at themidline to form the neural tube,and neural crest cells separatefrom the neural folds.

Neural grooveCrest of the neural fold

Neural fold

Crest of the neural foldNeural crest cells

Neural tube

Somite

Skin

Neural crest cells

Neural tubeNotochord

PROCESS FIGURE 29.7 Formation of the Neural Tube

The neural folds come together in the midline and fuse to form a neural tube. This fusion begins in the center and moves both cranially and caudally. Shownis an embryo at about 21 days after fertilization. The insets to the right show progressive closure of the neural tube at various levels of cross section.