Figure 1. Developmental stages of X. laevis and anatomical organization of the spinal motor columns.

(A) Relative size differences between stage 50 and 57 larvae (A₁) and associated morphological changes of the hindlimb bud during metamorphosis onset (A₂), from Nieuwkoop and Faber (1956). (B, C) Segmental (B₁, C) and rostrocaudal (B₂) organization of the spinal motor column region containing limb MNs at stages 50–52 (B₁, B₂) and 55 (C). Inset in B₂ shows the location of labeled MNs in the mid-region of the spinal cord. (D) Schematic representation of the segmental organization of the appendicular and axial motor columns in the larval spinal cord. Ap./Ax. MNs, appendicular/axial motoneurons; Ap./Ax. d., Ap./Ax. dendrites; Vr, ventral root; c.c, central canal; D, dorsal; L, lateral. Scale bars: A₁ and A₂ = 5 mm; B₁ and C = 20 µm; B₂ = 100 µm.