Start-up　flow　from　rest　in　cavities　plays　an　important　role　in　many　related　applications.　In　this　study,　flow　visualization　experiments　were　carried　out　to　investigate　the　effects　of　Reynolds　numbers　(Re　=　0-266)　and　cavity　aspect　ratios　(opening　width/cavity　diameter　of　1.1,　1.3,　1.5　and　1.7)　on　developing　separatrix　morphologies　in　round　microcavities.　Three　different　separatrixes　of　dye　solution　appear　successively　and　evolve　with　time　at　Re　=　155.　To　reveal　the　mechanism　of　separatrix　formation　and　migration,　the　flow　filed　characteristics　of　developing　vortex　structures　were　quantitatively　measured　using　micro-particle　image　velocimetry　(micro-PIV)　system.　Three　evolution　modes　of　the　separatrixes　were　mapped,　which　are　determined　by　the　combined　effects　of　flow　conditions,　cavity　aspect　ratios　and　the　cavity　opening　width　in　a　complex　way.　The　results　could　provide　deep　insights　into　the　physics　of　fluid　convection　and　mass　transfer　during　the　initial　flow　stage　in　round　microcavities　and　guide　the　design　of　devices　in　related　microfluidic　applications.　[GRAPHICS]　.