pseudoacephalus

An apparently headless placental parasitic twin that, however, has rudimentary cephalic structures that can be demonstrated by dissection. [pseudo- + G. a- priv. + kephale, head]

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pseu·do·aceph·a·lus (soo″do-a-sefґə-ləs) [pseudo- + acephalus] in asymmetrical conjoined twins, a parasitic twin, apparently headless, but with a rudimentary cranium contained in the more developed twin.