by 3rd day of development of egg days
||Chicken Gallus gallus
||Bradley M. Patten & Theodore C. Kramer, The initiation of contraction in the embryonic chick heart, Developmental Dynamics, November 1933 DOI: 10.1002/aja.1000530302 p.354 bottom paragraph
||Johnstone, P. N. Studies on the physiological anatomy of the embryonic heart. I. The demonstration of complete heart block in chick embryos during the second, third, and fourth days of incubation. Johns Hopkins Hosp. Bull., vol. 35, pp. 87-90. AND Johnstone, P. N. Studies on the physiological anatomy of the embryonic heart. II. An inquiry into the development of the heart beat in chick embryos including the development of irritability to electrical stimulation. Johns Hopkins Hosp. Bull., vol. 36, pp. 299-311.
||P.354 bottom paragraph: "In two striking papers (primary sources) Johnstone confirmed Sabin's observations that heart beats appeared first in the ventricle and showed also that by the third day of development, when the sino-atrial end of the heart had been definitely established, it assumed the lead in maintaining the cardiac rhythm."