implacentalWiktionary 原文构词拆分in-前缀in, into, towards, within.+placental词根/词干词源说明From im- + placental.释义（英文）形容词 Adjective(zoology, archaic) Not having a placenta (said of e.g. marsupials and monotremes).名词 Noun(zoology, archaic) A mammal having no placenta.