To develop a model for the migration of interneurons into the cerebral cortex, we placed hCS and hSS adjacent to each other inside a conical tube (Fig. 2a). We used day 60 hCS that resembled the mid-gestation pallium11, a developmental stage characterized by extensive migration of interneurons. After 3 days, the two spheroids fused (Fig. 2b). we used viral labeling of spheroids before assembly to monitor cell migration, and used a previously described DNA element near the Dlx1 and Dlx2 locus (Dlxi1/2b) that labels medial ganglionic eminences (MGE) and derivatives15,16. Approximately 65% of Dlxi1/2b::eGFP+ cells in hSS expressed GAD67 and contained GABA and markers for GABAergic neuron subtypes (Extended Data Fig. 5a–d). We then used live imaging to monitor the position of Dlxi1/2b::eGFP+ cells in fused hSS-hCS over multiple weeks. We observed a progressive movement of eGFP+ cells from hSS into hCS (Fig. 2c; Supplementary Video 1). This movement was specific to fused hSS-hCS and unidirectional: we observed minimal movement either from hCS into hSS in fused hSS-hCS or from hSS into hSS in fused hSS-hSS (Fig. 2d; Extended Data
