Formation of coronary arteries sprouting from the primitive aortic sinus wall of the chick embryo.

The formation of coronary arteries in chick embryos was observed by scanning electron microscopy on injected casts as well as by transmission electron microscopy. Usually, 2-4 primitive coronary arteries appear from the right aortic sinus below the level of the cusp margin, and 1-3 from the left one. As development proceeds, the arteries are generally reduced in number to form a single definitive coronary artery on each side. Canalization of the arteries seems to take place by partially degenerative changes of the primordia.