A human in situ placenta from the fourth lunar month of pregnancy has been cut in series and examined under the light microscope. The following results were found: Island-cells and septal cells above from Nitabuch's layer have identical histological characteristics. Placental septa and islands consist of trophocytes, fibrinoid and invaded portions of chorionic villi. They are partly or completely surrounded with syncytiotrophoblast. The islands and so called septa are not distributed evenly throughout the entire placenta. About 70% of these structures are in the medial area and the remaining 30% are in the peripheral area. At least 75% of the islands are not true islands, but after investigation of serial-sections, prove to be longitudinal or cross sections of completely bizarrely formed septa. The islands which have no contact to the septa appear to have connecting cell bridges to the chorionic villi and the chorionic plate. Because of the structure and distribution of the septa, it seems to be that they cannot have any significant hemodynamic function. No anatomical barrier, in the sense of a septum as a dividing wall, exists between the individual flowing units.