This study uses single-cell RNA sequencing to analyze the transcriptome dynamics from oocyte to morula in human and mouse embryos. The researchers identified novel stage-specific monoallelic expression patterns for a significant portion of polymorphic gene transcripts (25 to 53%). By performing weighted gene co-expression network analysis, they found that each developmental stage can be delineated by a small number of functional modules of co-expressed genes, indicating a sequential order of transcriptional changes in pathways of cell cycle, gene regulation, translation, and metabolism. Cross-species comparisons revealed that most human stage-specific modules are preserved in mice, but developmental specificity and timing differ between the two species. The study also identified conserved key members (or hub genes) of the human and mouse networks, which are likely to be crucial for driving mammalian pre-implantation development. These findings provide valuable insights into the genetic programs underlying early mammalian embryogenesis.This study uses single-cell RNA sequencing to analyze the transcriptome dynamics from oocyte to morula in human and mouse embryos. The researchers identified novel stage-specific monoallelic expression patterns for a significant portion of polymorphic gene transcripts (25 to 53%). By performing weighted gene co-expression network analysis, they found that each developmental stage can be delineated by a small number of functional modules of co-expressed genes, indicating a sequential order of transcriptional changes in pathways of cell cycle, gene regulation, translation, and metabolism. Cross-species comparisons revealed that most human stage-specific modules are preserved in mice, but developmental specificity and timing differ between the two species. The study also identified conserved key members (or hub genes) of the human and mouse networks, which are likely to be crucial for driving mammalian pre-implantation development. These findings provide valuable insights into the genetic programs underlying early mammalian embryogenesis.