Sequence of atrial depolarization at different stages of development of the chick embryo.

Simultaneous left and right atrial surface electrograms were obtained from the right and left atria of 30 chick embryos. These were divided into 3 equal groups that were studied at 15 to 16, 10 to 11, and 5 to 7 days of incubation. The embryos were removed from the shell with their circulation intact and temperature maintained constant. In an early stage of development (5 to 6 days), the left atrium and its appendage were larger than the right, and left preceded right atrial depolarization. At and after 7 days of incubation, the right atrium and its appendage were larger than the left, and right preceded left atrial depolarization. The intervals for interatrial conduction and for atrioventricular conduction were about the same at all stages of development. Since the length and width of the heart at 15 to 16 days of incubation were about 3 times those of a heart at 5 to 7 days of incubation, it is proposed that the higher the stage of development of the chick embryo the greater must be the conduction velocity between the two atria and from atrium to ventricle. This was assumed to be related to the degree of development of the specialized conduction pathways that are probably absent or underdeveloped in early stages of development. ADDITIONAL