Trunk formation in a dish Building mammalian embryos from self-organizing stem cells in culture would accelerate the investigation of morphogenetic and differentiation processes that shape the body plan. Veenvliet et al. report a method for generating embryonic trunk-like structures (TLSs) with a neural tube, somites, and gut by embedding mouse embryonic stem cell aggregates in an extracellular matrix surrogate. Live imaging and comparative single-cell transcriptomics indicate that TLS formation is analogous to mouse development. TLSs therefore provide a scalable, tractable, and accessible high-throughput platform for decoding mammalian embryogenesis at a high level of resolution. Science , this issue p. eaba4937