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Fig. 1 The posterior commissure (PC) and the medial longitudinal fasciculus (MLF) are present but modified in noi mutant embryos. Immunochemical labelling of acetylated tubulin was used to study axon formation in 48 hours (48h) postfertilization embryos, oriented anterior to the left. A,B: A lateral view of whole-mounted embryos. The tectum and the cerebellar fold, including their neurons, e.g., the nervus trochlearis, are absent in the mutant. Arrowheads in A and B mark the layer of the dorsal section in C and D, respectively. C,D: The PC, which demarcates the diencephalic/mesencephalic boundary, is still present. The PC looks more condensed in the mediolateral direction (arrowheads) and hindbrain neurons, visualized in small dots expand anteriorly (arrows). E,F: Higher magnification of the lateral view of the mutant embryo shows additional axons emerging from the dorsal midbrain area (arrowheads) and joining the medial longitudinal fasciculus (MLF). In the anteroposterior direction, the PC appears larger and defasciculated (E,F; arrows). AC, anterior commissure; DVDT, dorsoventral diencephalic tract; mhb, midbrain-hindbrain boundary; tPC, tract of the posterior commissure; POC, postoptic commissure; tec, tectum opticum; TN, telencephalic nucleus; TRI, trigeminal nerve, tro, trochlear nerve; wt, wild-type.