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 Fig. 5e, f). The same pattern of migration could be observed for hSS-hCS assembled at later stages (Extended Data Fig. 5g). When hSS were plated on a coverslip, the migration was inefficient or absent (Extended Data Fig. 5h–j; Supplementary Video 2) similar to rodent cultures17. In the first 10 days after assembly, the vast majority of Dlxi1/2b::eGFP+ cells that migrated away from hSS had the leading process positioned towards hCS at either a 45° or 90° angle relative to the interface (Extended Data Fig. 5k). At 30–50 days after asssembly, 60% of the migrated cells were localized within the outer 100 μm of hCS (Extended Data Fig. 5l), and a large population of interneurons migrated into hCS as shown by optical clearing (Fig. 2e). Interestingly, we also observed processes of Dlxi1/2b::eGFP+ cells that briefly touched VZ-like regions, reminiscent of rodent ventricle-directed migration18 (Supplementary Video 3; Extended Data Fig. 5m–o). We next investigated the fate of Dlxi1/2b::eGFP+ cells in hSS after
