Focal presence of the embryonic pattern of myoarchitecture and of a lacunary blood supply was found in the left ventricular wall of five infant hearts. Four of these hearts showed various malformations; one was a case of cardiac fibroma. The persisting intertrabecular spaces and sinusoids communicated with the ventricular lumen; there appeared to be some communication with the coronary branches. The intertrabecular spaces of the spongy myocardium were lined with a continuous layer of endothelial cells, thus resembling the microscopical appearance of myocardium of adult cold-blooded vertebrates rather than the embryonic phase of myocardial development of warm-blooded animals.