This scRNA-seq data is an integral part of a manuscript with the above title. Using computational methods, we were able to reconstruct a detailed branched trajectory reflecting pancreatic endocrine differentiation in the mouse embryo. Analysis of the transcriptional changes occuring during the differentiation suggested that epithelial-to-mesenchymal transition likely plays no role in this process, contrary to the prevailing dogma. Our findings were corroborated with high-resolution imaging of the developing pancreas, revealing how differentiating endocrine progenitors migrate in cohesion, forming bud-like islet precursors, or "peninsulas", and that spatiotemporal collinearity during differentiation leads to the typical core-mantle architecture of the mature, spherical islet. This work led to a complete overhaul of our understanding of how pancreatic islets are developed, laying the ground for the generation of entire islets in vitro as a potential novel source of islet transplantation. Overall design: Single-cell suspensions were prepared from pancreata of Neurogenin 3-eGFP mouse embryos sacrificed at different days of embryonic development. Single eGFP-positive cells were FACS-sorted into 96-well plates, and single-cell cDNA was prepared using the SMART-seq protocol. Single-cell sequencing libraries were generated using the Nextera XT DNA library preparation kit and sequenced on an Illumina HiSeq sequencer. Reads were aligned to the mouse reference genome build mm10 with TopHat, and single-cell gene expression profiles were computed using Cufflinks.