Round structure consisting of chorionic villi[PMID]. A separation of the decidua basalis of the placenta, separated by placental septa. Each cotyledon consists of a main stem of a chorionic villus as well as its branches and subbranches. The cotyledons receive fetal blood from chorionic vessels, which branch off cotyledon vessels into the cotyledons, which, in turn, branch into capillaries. The cotyledons are surrounded by maternal blood, which can exchange oxygen and nutrients with the fetal blood in the capillaries[WP]. [ http://en.wikipedia.org/wiki/Placental_cotyledon http://www.ncbi.nlm.nih.gov/pubmed/19829370 ]