Ovule.—The seed in its earliest condition.
Pangenesis (Darwin).—The theory of Heredity by gemmules (q. v.).
Panmixia (Weismann).—The condition of free intercrossing, i.e. where Natural Selection (q. v.) cannot act.
Parthenogenesis.—A degenerate form of sexual reproduction, in which the egg develops without having been fertilized by the male element.
Phylogeny.—The ancestral history of the race, as distinguished from the life history of the individual (Ontogeny).
Physiological Units (Spencer).—Special units which it is inferred a plant or animal of any species is made up of, and in all of which dwells the intrinsic aptitude to aggregate into the form of that species.
Plasma.—The constituent material of cells, e. g. germ-plasma (of sexual-cells), somatoplasma (of body-cells).
Plasmogenetic characters.—Variations due to admixtures of germ-plasm in acts of sexual fertilization (and therefore present at birth), as distinguished from somatogenetic characters—variations which have been acquired independently of germ-plasm. See Somatogenetic characters.
Polar bodies.—Before an egg is fertilized the nucleus moves towards the periphery and divides twice. The two cells that are thus formed are the polar bodies. The extrusion of polar bodies is probably universal among animals, but only one polar body is extruded from parthenogenetic ova. See Darwin and after Darwin, pp. 125 and 126.