Formation of the cellular blastoderm is followed by gastrula-tion, the process of cellular invagination that results in the formation of a layered embryo comprising two germ layers. Cells that remain at the blastoderm periphery will form the ectoderm, and cells that invaginate below the ectoderm will form the mesoderm. The presumptive mesoderm in most species consists of a strip of cells along the ventral midline (Fig. 1). Gastrulation can happen in any number of ways: by the mesodermal cells invaginating simultaneously, as in Drosophila, or by cells invaginating sequentially, beginning at the anterior, while the gastrulation furrow progresses toward the posterior. Most often the presumptive mesoderm lies along the ventral midline, but in the apterygote thysanuran Thermobia domestica, cells migrate inward from every part of the germ band. Regardless of the mechanism of gastrulation, the end result is a bilayered embryo, with mesodermal precursors underlying the ectoderm.
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Make money with honey How to be a Beekeeper. Beekeeping can be a fascinating hobby or you can turn it into a lucrative business. The choice is yours. You need to know some basics to help you get started. The equipment needed to be a beekeeper. Where can you find the equipment you need? The best location for the hives. You can't just put bees in any spot. What needs to be considered when picking the location for your bees?