Only blastoderm cells destined to form the embryo coalesce to form the germ anlage, which later develops into the germ band. The cells that do not contribute to the germ anlage form an extraembryonic membrane called the serosa. In most species, the boundary between the future serosa and the future embryo ruptures, and the serosal cells migrate over and envelope the embryonic primordium and yolk cells (Fig. 1). However, there is variation in how the serosa is formed. In extreme cases like dipterans, the serosa cells do not migrate over the germ anlage but remain as a cluster of cells on one side of the egg. In addition to the serosa, a second protective membrane, the amnion, forms later from the cells immediately adjacent to the germ anlage. These cells proliferate, flatten, and elongate. As they extend over the germ band, they resemble a sleeping bag being pulled up from the posterior and down from the presumptive head lobes of the germ band. Ultimately, the amnion cells meet in the middle of the embryo and form a single cell layer that lies between the embryo and the now separate serosa. In some derived, holometabolous species (e.g., Drosophila), this membrane has become vestigial, and the cells never migrate over the germ band.
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