Examination of the lipid-cytoplasm interface of the weakly osmiophilic fat droplets in developing adipose cells from chick bone marrow reveals that the lipid does not lie free in the cytoplasm, nor is it bounded by a membrane, but instead is encompassed by a highly ordered complex of small, 80 Å thick, filaments.

In oblique sections, superimposition of the images of adjacent filaments results in a more or less continuous line which resembles a membrane. Perhaps this view of the system of filaments, in sections where heavily osmicated fat had retracted slightly from the interface, is responsible for reports that the lipid in adipose cells is enclosed by a membrane.

It is suggested that a similar system of filaments may be visualized in adipose cells of other species after extraction of the strongly osmiophilic fat which obscures details at the surface of the lipid droplets.

Although the true functional significance of the filaments is not yet known, the morphological evidence alone suggests that they may provide support for the lipid inclusions. An interpretation of the geometry of the filaments, consistent with the present evidence, is presented.