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The partition that separates the right and left ventricles from each other is called interventricular septum (or just ventricular septum).
The interventricular septum is a composite septum which consists of two parts: (1) a thicker muscular part called septum musculare, and (2) a thin membranous part known as septurn membranaceum. Development of these two parts occurs from different sources.
Initially the primitive right and left ventricles do not have any partition be­tween them. At the end of the 4th week the two primitive ventricles begins to grow rapidly. Consequently, the -medial walls of the expanding ventricles be­come apposed to produce a thick mus­cular ridge which is the primordium of the muscular interventricular septum. This septum grows further by the active proliferation of myoblasts in it. However, the growth of this septum stops; in the middle of the seventh week and there remains a crescentic. gap between the septum intermedium and concave free edge of the septum mus­culare. This gap, called interventricular foramen, permits communication between the right and left ventricles.
• Closure of the interventricular foramen occurs during the seventh week by the formation of the septumj membranaceum. This septum is formed mainly by tissue derived from the inferior endocardial cushion (which has by now joined the superior endocardial cushion to form the septum in­termedium).  Mostauthorities  believe that the right and left bulbar ridges (de­veloping in the conus cordis) also con­tribute tissue to the septum membra­naceum. The newly formed tissue grows to give rise to a thin, membra­nous septum which fuses with the concave upper edge of the septum muscu­lare and, thus, closes the interventricu­lar foramen. Closure of this foramen 1 not only shuts off the communication between the right and left ventricles but also ensures that the right ventricular cavity communicates with the pulmo­nary trunk and the left ventricular cavity communicates with the aorta. In addi­tion, the right atrioventricular opening now connects exclusively with the right ventricular cavity, and the left atrioven­tricular opening with the left ventricular cavity.
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