The Intrinsic Pulsation Rates O F Frag- Ments O F the Embryonic Chick Heart

It has been shown previously that the pulsation frequency of the heart of the embryonic chick increases rapidly during the first hours after its activity begins. Ordinarily the first heart beats occur a t about the twenty-ninth to the thirtieth hours of incubation. During the next few hours the acceleration is at first rapid, but thereafter declines. By the fiftieth hour of incubation the rate of acceleration (not the rate of beating) has become markedly less (Barry, '40). The present investigation is the first step in an attempt to determine the causes of this phenomenon. Since the heart is spontaneously rhythmic, its rate must be fundamentally determined by the inherent pulsation frequency of some intrinsic pacemaking mechanism. This basic rate may be altered by secondary factors in the immediate environment of the myocardium. The data presented here establish the inherent rhythmicity of the myocardium at various cardiac levels and at various ages of the embryonic heart. Only after such information is available can the effect of secopdary modifying factors be evaluated. The heart is formed by the successive addition of myocardial tissue to its caudal end (Patten, '22). Since each