In accordance with expectation, the embryonic chick heart exposes recurrently temperature characteristics of the orders 8,200, 14,200, 16,400, 18,300, 20,500 and 25,200. These are found in both myogenic and neurogenic hearts but not with the same frequencies. These two periods of development also differ in the localisation of critical points. So far µ = 8,200, 14,200 and 18,300 have been exposed in embryos only by Fundulus; the remaining three orders however are recurrent also in the embryogeny of Limulus. Two orders not included in this hst (5,000 and 11,000) are rare, yet, even so, not restricted to any one of the three forms. These facts, together with their implications, suggest for heart rhythm in Limulus, Fundulus and the Chick, an underlying physical-chemical mechanism in structure identical for all three types of heart at all stages of development, and differing in neurogenic stages only by a revision of the myogenic time relations.