During developmental period around the sixth week of gestation lymphatic channels are formed from a series of clefts that develop in the mesenchyme. From these channels, sacs are formed that establish drainage with the venous system. Failure to establish venous drainage results in dilated disorganized lymph channels, which, in the largest form, present as cystic hygromas. The masses are usually large soft structures without clear margins. The most important sign is the illumination test.