Fig. 2. Transplantation experiments. (A-D) Paramedial (type 3) grafts. (A-C) Grafts placed close to the midline (type 3a and 3b) are found elongated bordering on the midline. When placed at the isthmus (type 3a), they end up either in the midbrain (A) or straddling the MHB (B), while when placed rostral to the isthmus (type 3b), they become elongated in the caudal midbrain (C). (D) Grafts placed rostral to the isthmus and at a distance from the midline (type 3c) move rostromedially in the midbrain but are no longer elongated and have increased in size relatively to the grafts placed closer to the midline. (E-H) Bilateral dorsal strip (type 4) grafts of the caudal midbrain move rostromedially but also caudally in the hindbrain where they contribute to thin medial streams of cells that avoid the roof plate. (I,J) Small midline (type 5) grafts. Small grafts centered on the isthmus contribute modestly to the midline either exclusively in the hindbrain (I; flat-mount view of the grafted embryo) or straddling the MHB (J; ibid.).

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