Fig. 3. Regulatory gene expression domains and positions of reticulospinal neurons. (A,B) Neuronal labeling in combination with whole-mount in situ hybridization using a LjKrox20 riboprobe. (A) Lateral view of a stage 25 embryo. With respect to the LjKrox20 expression domains, the I4 neuron is located in r3, Mth neuron in r4, and Mth' neuron in r5. (B) Lateral view of a stage 26 embryo. Newly developed I3 neuron is located in putative r2. Note that other labeled neurons are located at the same axial levels as in the stage 25 embryo shown in A. (C,D) Reticulospinal neurons and LjHox3 expression. (C) Lateral view of a stage 25 embryo. (D) Ventral view of a stage 26 embryo. The position of the Mth neuron corresponds to the anterior LjHox3 expression boundary (arrowheads).