part of part of in taxon develops from develops from composed primarily of spongiotrophoblast cell Rodentia cell layer placenta spongiotrophoblast layer spongiotrophoblast layer of placenta BTO:0005402 EMAPA:31876 UBERON:0004021 The structure of the outer structural layer of the rodent placenta[MP]. spongiotrophoblast ectoplacental cone reproductive structure extraembryonic tissue