Cystic Hygroma
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.