part of part of starts embryo stage part embryo stage life cycle zygote stage 1-cell stage A stage at which the organism is a single cell produced by means of sexual reproduction. As in all metazoans, eumetazoan development begins with a fertilized egg, or zygote.[well established][VHOG] BILS:0000106 BilaDO:0000005 EFO:0001322 EHDAA:27 EMAPA:16033 FBdv:00005288 PdumDv:0000100 UBERON:0000106 VHOG:0000745 XAO:1000001 ZFS:0000001 fertilized egg stage fertilized egg stage one cell stage one-cell stage uberon zygote zygotum