The development of the atrioventricular bundle and its branches in the avian heart.

A light microscopic study of serial sections of the hearts of 64 avian embryos (chicken, turkey, duck and goose) was undertaken in order to elucidate the development of the atrioventricular bundle and its branches. The developing atrioventricular bundle and its branches were shown to appear simultaneously (at about a third of the way through incubation) as relatively large, discrete fasciculi, which were dark-staining when compared with developing cardiac muscle. Prior to the appearance of the bundle and branches, the developing conducting tissue was found to be present as a widespread precursor tissue, which was distributed as a complete, dark-staining subendocardial sleeve, lining the whole of the ventricular canal, ventriculobulbar loop, and extending into the common atrial cavity. From these observations, it is concluded that, with septation and chamber formation, the tissue of the sleeve is partly incorporated within the chamber walls to give origin to diffuse Purkinje tissue, and is partly retained as the bundle and branches. The manner of derivation of the bundle and branches is discussed, and it is stressed that the definitive positions of the bundle and branches denote the junctional zones of the components of septation.