Vesicles undergoing growth and division in the laboratory. Vesicles, fluid-filled cavities or sacs enclosed by lipid molecules (green circle in the first panel), can be made from fatty acids. Fatty acids in aqueous solution are organized as spherical droplets called micelles. As the external concentration of fatty acids increases (more local micelles), the mostly spherical vesicles (top) grow slowly into a filamentous form (bottom) by incorporation of the added fatty acids. The micelle concentration declines as the micelles are incorporated into the larger vesicles. Gentle agitation produces a solution that again consists of mainly spherical vesicles, as shown in the schematic.