The Germ Band And Dorsal Closure

The germ band is a two-layered structure, comprising both ectoderm and mesoderm, that represents the outline of the final body plan along both axes. As the embryo grows, the germ band transforms from this essentially two-dimensional, two-layered sheet into a three-dimensional larva. From anterior to posterior, all the segments are represented. Individual segments first become visible near the anterior end, where the ectoderm differentiates into the brain and compound eyes. Protrusions develop anterior to the mouth opening that will eventually grow to form the labrum (front lip of mouthparts) and the antennae. The next segment, the intercalary segment, develops a transient limb bud, which is later retracted. This bud may be a remnant of a second pair of antennae found in this position along the anterior—posterior axis in crustaceans. Each of the first three segments behind the mouth form paired appendages that become the mouthparts: mandibles, maxillae, and labium. The next three segments

FIGURE 4 Comparative germ band and expression patterns of segmentation genes. [Modified from Nagy, L. (1994). Curr. Biol. 4, 811—814.]

develop into the thorax and form appendages that become walking legs. During the remainder of embryogenesis, as organs develop and differentiate, the flanks of the germ band, both ectoderm and mesoderm, grow laterally and extend around the yolk. The two edges of the germ band meet and fuse along the dorsal midline, such that the mesodermal and ectodermal layers now enclose the yolk.

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