otp1 expression in the forebrain. Anterior is always to the left. Except I, a ventral view, right panels are lateral views and left ones are dorsal. Eyes have been removed in Iand J. A, B, About 1 h after early otp1 expression in the hindbrain (11 hpf, see Fig. 4A), a small lateral domain appears at the anterior edge of the central nervous system (arrow) (see also Fig. 6A). Shortly after, a novel center of transcription emerges posterolaterally in the alar plate (arrowhead). C, D, Then, otp1 expression diffuses posteriorly in a segmented fashion with three clusters (1–3) (16.5 hpf). During this developmental phase, spatial rearrangements of the anterior forebrain start occurring as the result of the ventral bending of the neuraxis . Even though intrinsic topological changes in otp1 transcription may also happen, their extents are eventually obscured by morphogenetic dislocations in the neural tube. E, F, At 19 hpf, otp1 positive cells in the middle of longitudinal domains displace dorsally with respect to cells at different axial positions. Consequently, otp1 labelling in the anterior forebrain coalesces in the proximity of the optic recess (arrow). G, H, 28 hpf WISH with a large compacted anterior diencephalic cluster in PO and a small one caudally in PT. I, J, Later in development (72 hpf), otp1 expression is detected in three distinct domains (1–3) in the ventral diencephalon. Unlike early in development, these three clusters are fused along the midline. ce, cerebellum; dt, dorsal thalamus; fb, forebrain; h, hypothalamus; is, isthmus; mb, midbrain; P1, pharyngeal arch 1; po, preoptic area; pt, posterior tuberculum; t, telencephalon; tc, tectum; tg, tegmentum.