A human embryo of 11 mm crown-rump length (six weeks, stage 15) is described, in which a muscular defect is present in the ventricular septum, in addition to the interventricular communication normal for this stage of development. The defect consists of large confluent spaces between very loose trabeculations. Its morphology is identical to that of the central muscular ventricular septal defect well recognized in the mature heart. It is suggested that the defect was caused by widening of the sinusoidal spaces which are temporarily present in the normal developing septum.
