Complex sentence: As blastoderm cells migrate anteriorly and push primary hypoblast cells, forming a secondary hypoblast known as the endoblast, Koller's sickle is responsible for preventing the hypoblast cells and the area opaca cells from making contact with the blastoderm, which allows the primitive streak to form.
Split into simpler sentences: 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.