result of this is that the inferior opposing walls of the ventricles
give the appearance of an invagination between the ventricular chambers.
The fusion of the opposing ventricular walls then gives rise to the muscular
interventricular septum.
The development of the interventricular septum begins around the 27th
day of gestation. At this time, the ventricular cavities are small and
communicate with each other by a narrow channel. The ventricles grow and
enlarge rapidly while the communicating channel grows slowly. The
result of this is that the inferior opposing walls of the ventricles
give the appearance of an invagination between the ventricular chambers.
The fusion of the opposing ventricular walls then gives rise to the muscular
interventricular septum.
The interventricular septum develops from three embryonic components:
the 
endocardial cushions, the 
conus cushions and the muscular septum. The endocardial cushions divide
the inflow tracts from the ventricles. The conus cushions divide the outflow
region of the ventricles. The 
truncus cushions divide the truncus arteriosus in the aorta and main
pulmonary artery.