Precursors of the fat body (fb) split from the inner layer of the mesoderm (shaded gray) around stages 11 and 12. They lie close to, yet more internal than, the somatic mesoderm cells (sm), giving rise to the ventral and lateral somatic muscles (see Bate, this volume).

During stage 13, the column of fat body precursors becomes an elongated sheet of cells sandwiched between the developing visceral musculature (vm) and the body wall. Some regional specializations of the fat body become apparent during these stages: Dorsally, a narrow strip of cells (dfb) separates from the main mass of fat body. Similarly, anteriorly and posteriorly, 'horizontal plates' of fat body (apl, ppl) arise beneath the foregut and hindgut, respectively.

During stage 17, large holes and clefts appear in the formerly solid sheet of fat body cells. These holes appear at locations where other organs (i.e., trachea, gonads) push into the fat body.
(mg) Midgut;
(mp) Malpighian tubules;
(myo) myoblasts.