AbstractEach leg of a crayfish embodies two types of morphogenetic fields, a limb field and a sequence of segmental fields. These fields were manifest in grafting operations along the proximodistal axis of legs 3 and 4. The propodite was grafted into more proximal segments—carpopodite, meropodite, or basipodite—in one set of operations. In a second set the propodite was host for a graft from the propodite or carpopodite. After these operations intercalary regeneration yielded complete regenerates, which manifested the limb field, and incomplete regenerates, which showed the segmental field. Complete legs contained all structures present in a normal leg. After some operations complete legs also had supernumerary structures. Several types of incomplete legs regenerated—(1) legs with fusion of the host and graft segments; (2) legs with a joint girdling only part of the leg circumference; and (3) legs with one mosaic segment intercalated to replace two deleted segments. These results show that both limb and segmental fields control intercalation in the tested segments of crayfish legs, as in insect legs. We propose a “mode control hypothesis” to describe the way in which molt cycle phase and healing rate bias the type of field controlling regeneration. Our observations also provide clues about the operation of the fields to generate structures. Homologous parts of the segmental field do not necessarily generate analogous structures in different leg segments. Although intersegmental membrane often develops in conjunction with an apodeme (muscle tendon), in a mosaic joint intersegmental membrane can still develop in the absence of the corresponding apodeme. In a partial joint the length of the segments adjacent to the joint is subnormal, but greater than the length of the segments between which a joint is missing. The jointless region may limit the growth of the joint