During cerebellar development in mammals and birds, two lateral primordia fuse on the dorsal midline of the neural tube to form the vermis. On p. 5319, Louvi et al. show that cells from the isthmus – a domain at the mid/hindbrain junction of the developing neural tube – are essential for this fusion. Their fate-mapping experiments in avian embryos, and analyses of cerebellar fusion in wild-type and mutant mouse embryos, reveal that the isthmus-derived cells provide a cell substratum in which fusion can occur, and also act as a source of signals needed to modify the edges of the primordia to allow fusion. In an extension of this work, Alexandre and Wassef identify a restricted dorsal domain of the isthmic organiser that is involved in the formation and positioning of the roof plate in the avian caudal midbrain (see p. 5331). They suggest that this isthmic node is analogous to the Hensen's node region that is required for floor-plate formation.