Plantigrade: Walking on the whole sole of the foot, like bears.
Plastids: Permanent organelles or living components of the cellular cytoplasm, e.g., chloroplasts, leucoplasts, etc.
Pleistocene: The lower series of the Quaternary system of fossiliferous rocks. It corresponds to the so-called Glacial epoch, and extends from the close of the Tertiary period (system) to the dawn of the Recent or Historical epoch.
Polar Cell: A synonym for polar body, or policyte. The polar bodies are minute abortive cells given off by the egg undergoing meiosis. Into them are shunted the chromosomes which the egg discards in its process of nuclear reduction (maturation).
Præformation: Theory that the egg contains a complete miniature of the organism into which it develops.
Prehension: Grasping, catching hold.
Progression: Advancing movement, locomotion.
Pro-simiæ: The lemurs as distinguished from genuine apes (Simiæ).
Protista: Animals or plants which are normally unicellular and which when multicellular show no differentiation into tissues—the Protozoans and Protophytes.
Protoplasm: Living matter.