Schematic view of the embedded disk within a collapsing envelope. Unlike in traditional protoplanetary disks, i.e. Class II objects, embedded disks (Class O/I) are continually fed with new gas and dust from the envelope (infall). The gas in the embedded disk is subject to turbulent flow and evolves viscously. The dust, on the other hand, tends to sink to the midplane of the embedded disk and radially drifts towards the host star. Because the gas, but not the dust, is held aloft by gas pressure, the gas and dust arriving at the embedded disk at any given time originate from different initial locations in the envelope; dust arrives from larger initial radii than gas. The grey scale broadly shows the total volume mass density of the gas and dust in the protostellar system.