Closure of the interventricular foramen and morphogenesis of the membranous septum and ventricular septal defects in the human heart.

The development of the atrioventricular and interventricular membranous septum was studied The interventricular foramen is closed by the growth of the endocardial crest of the interventricular muscular septum towards the cranio-ventral atrioventricular cushion and towards the part B of the outflow tract ridge B-3; the bulbus is septated by the apposition and fusion of parts A and B of the outflow tract ridges A-1 and B-3. The membranous septum develops from the crest of the interventricular muscular septum between the interventricular muscular septum and the right border of the atrioventricular cushions, where it forms the atrioventricular mesenchymal septum. This last develops towards the right surface of the interventricular muscular septum and forms the septal cusp primordium of he tricuspid valve; this is undermined, for which reason the septal cusp is cut off from the interventricular muscular septum at the 5th month of fetal life. From the 6th month of intrauterine life, the undermining extends to the atrioventricular mesenchymal septum, for which reason the right septal cusp becomes inserted in the right face of the atrioventricular mesenchymal septum which becomes divided into atrioventricular and interventricular. The Authors advance some hypotheses as to the origin of certain defects of the interventricular septum.