Embryos were collected at noon of experimentally-determined timepoints in ice-cold phosphate-buffered saline (PBS) under a Leica MZ6 dissecting scope. Atrioventricular canals (AVCs) were dissected in PBS with fine dissecting forceps, ensuring removal of cardiac chambers and outflow tracts. AVCs were washed in PBS, and incubated for 5-15 minutes in TrypLE express at 37 degrees Celsius to dissociate tissues into single-cells. As per 10X genomics manufacturer's protocol As per 10X genomics manufacturer's protocol