Abstract
The QRS complex in A-B lead of an animal varied specifically in wave form from one developing stage of the animal to another. In the caprine fetus and chick embryo they showed completely the same wave form as in the respective adult, the rS type, in the second half of fetal life. In the dog, however, they varied in a different manner, exhibiting the same wave form as those in the adult dog, the Rs type, in the early neonatal stage. The present paper deals with the relationships among the alteration in the wave form of the QRS complexes, morphological changes in the heart, and the formation of specific cardiac muscle in the ventricles in the fetal and neonatal stages. It was presumed that the alteration in the wave form of the QRS complexes might have been caused by quantitative changes in the relation to the ratio of right to left ventricular mass in neonatal dogs, and by qualitative changes in the penetration of Purkinje fibers into the free ventricular walls in chick embryos and caprine fetuses. The most marked difference in the altering process in the wave form of QRS complex between the two groups observed might be related to the penetrating stage of Purkinje fibers into the outer layer of the free ventricular wall.
