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 five minutes at 37 degrees Celsius in 1ml PBS, 10ug/ml DNAse I, 4mM EDTA, and 2% fetal bovine serum (FBS) (PDEF) with dispase/collagenase (2.5uU/ml dispase, 0.0025% collagenase) to dissociate tissues into single-cells. As per 10X genomics manufacturer's protocol As per 10X genomics manufacturer's protocol