Which of these is a finger-like sac that has no known function in humans?

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..

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.