It is a membranous sac attached to an embryo, functioning as the circulatory system of the human embryo before internal circulation begins until about 3 weeks of age if I recall. It also forms the umbilical cord. A collet is used for clamping purposes. It forms a collar around an object, and clamps down on it as it is tightened. A 5C collet has external threads used for lathes..
