The thickening of the epiblast in Koller's sickle acts as a margin separating sheets of cells from posterior side of avian blastoderms from hypoblasts and area opaca endoderm. The blastoderm is a single layer of cells, and the hypoblast and area opaca endoderm cells lie directly below the blastoderm. Koller's sickle arises from the midpoint, between the hypoblast cells and the area opaca endoderm. As blastoderm cells migrate anteriorly they push primary hypoblast cells and form a secondary hypoblast known as the endoblast. Also during this migration, Koller's sickle prevents the hypoblast cells and the area opaca cells from making contact with the blastoderm, which allows the primitive streak to form.Carlson, Bruce M. Human Embryology & Developmental Biology. St. Louis: Mosby, 1999. Print.Society for Developmental Biology. "Developmental Biology." Symposium of the Society for Developmental Biology (1959): n. pag. Print.
