When the germ band is fully segmented and gastrulation is complete, the remainder of embryogenesis involves the differentiation of the ectoderm and mesoderm into the organ systems of the larva or juvenile. The ectoderm gives rise to the bulk of the larval or adult form. Most obviously the ectoderm forms the "skin" of the larvae, marked by numerous bristles and hairs. In addition, the nervous system develops from the ventral ectoderm, and the tracheal system develops from invaginations of the lateral ectoderm. Ocelli, salivary glands, a prothoracic gland, corpora allata, molting glands, oenocytes, and silk glands also develop as ectodermal invaginations. Finally, two additional invaginations of the ectoderm occur:
1. The stomadeum occurs in a central position near the anterior of the germ band, and once invaginated, these cells proliferate in a posterior direction to form the foregut.
2. The proctodeal invagination occurs in the terminal segment, and these cells grow anteriorly to form the hindgut.
Malpighian tubules, the insect excretory organ, develop from outpocketings of the proctodeum. The invaginated mesoderm initially forms a pair of transient coelomic sacs in each segment (Fig. 1E). From these, the dorsal vessel, or heart, the internal reproductive organs, muscles, fat body, subesophageal gland, and hemocytes will form. The midgut arises from a third germ layer, the endoderm, that develops at the edge of the fore- and hindgut invaginations and eventually fuses with them to complete the gut. During the remainder of development both the mesodermal and ectodermal organ primordia all undergo differentiation into tissue-specific cell types and cell rearrangements required to form the final organ structures.
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