Human pluripotent stem cells (hPSCs) offer a unique cellular model to study lineage specifications of the primary germ layers during human development. We profiled single-cell RNA-seq (scRNA-seq) on four lineage-specific progenitor cells derived from hESCs. Our scRNA-seq analyses revealed each type of progenitors display various extend of heterogeneity. Specifically, definitive endoderm cells (DECs) not only show a greater degree of heterogeneity, but are also enriched in metabolic signatures. Followed by detailed temporal scRNA-seq profiling along DEC differentiation, we reconstructed a differentiation trajectory using a novel statistical pipeline named Wave-Crest. Wave-Crest further identifies candidate regulators during the transitioning phase from Brachyury (T)+ mesendoderm towards CXCR4+ DEC state. To functionally test identified novel regulators; we generated a live cell monitoring system, a T-2A-EGFP knock-in reporter cell line via CRISPR/CAS9. We demonstrated that, among the top candidate genes, KLF8 plays a pivotal role modulating mesendoderm to DEC differentiation. In this submission, 1810 raw fastq files are provided; 212 are re-analysis from GSE64016. Four expected count matrices are provided - 1) 1018 single cells from snapshot progenitors; 2) 758 single cells from time couse profiling; 3) 19 bulk RNA-seq sample from snapshot progenitors; 4) 15 bulk RNA-seq sample from time course profiling. Overall design: Total 1018 single cells from snapshot progenitors and 758 single cells from time couse profiling. Matchd population bulk RNA-seq samples for both the progenitors snapshot (19 samples) and time course profiling (15 samples) also included in this submission. These data set are used to detect the transitioning phase from mesendoderm to definitive endoderm.