A falcine sinus that does not involute but persists beyond birth is defined as a persistent falcine sinus. In the 20-mm embryo stage, the primitive falx cerebri contains the sagittal plexus, a meshwork of anastomotic venous channels from which the superior sagittal sinus and straight sinus are thought to develop. A persistent falcine sinus represents a persistent channel of the caudal anastomotic loops of the sagittal plexus. [from HPO]