zonary placenta

A placenta in which the chorionic villi have aggregated to form a broad band that circles about the center of the chorion. Such zones may be complete circles (such as those in dogs and cats) or incomplete (such as those in bears and seals). It is thought that zonary placentae form from diffuse placentae in which the villi at the ends regress, leaving only those in the center to function. At the edges of the zonary placenta is the hemophagous organ, which is green. The color is due to the degradation of hemoglobin into bilivirdin. This green organ provides iron for the developing fetus.