Koller s sickle is one of two regions ( the other being the caudal boundary region of the area opaca ) where expression patterns for genes important for gastrulation are localized.
2.
Koller's sickle, which is a local thickening of epiblast involved in the formation of the primitive streak through which gastrulation occurs, separates sheets of cells from the posterior end of the blastoderm from the underlying hypoderm and area opaca endoderm.
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