Development of the pars membranacea septi interventricularis of the human heart. II. Thickness change.

The measurement of the thickness of the pars membranacea septi interventricularis of the human heart increases with the intra-uterine age of the fetus along a steep, ascending, straight line. The thickening of the PMSI occurs in phase I of the development from 0.246 mm (CR = 16.8 mm) to 0.524 mm (CR = 119 mm) and in phase II from 0.601 mm (CR = 149 mm) to 1.024 mm (CR = 362 mm). The thickness increases, then, from the ending of the closure of the secondary interventricular foramen to the end of the fetal period.