=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. =Preformation.=—The old conjecture (1672—Malpighi) that the development of an embryo was merely the expansion or unfolding of a miniature of the adult within the egg. =Protophyta.=—Unicellular plants (q. v.). =Protoplasm.=—Living matter. =Protozoa.=—Unicellular animals (q. v.). =Representative Congenital characters.=—_See_ p. 65. =Reversion.=—_See_ Atavism. =Rudimentary Organs.=—Usually considered a synonym of the term “vestigial characters,” and is the name under which are included all those organs which, either from having become useless or from other causes, have been much reduced in size, e.g. the muscles of the external ear in man (see _Darwin and after Darwin_, p. 76), &c. Latterly the former expression has been used to describe organs in process of development (e.g. the electric organ of the skate—_loc.cit._, p. 365 _et seq._), whilst the latter is made to embrace all those organs in process of elimination. =Soma.=—A general term descriptive of the whole mass of the body-cells of an organism. =Somatic-idio-plasm.=—_See_ p. 32. =Somatogenetic characters.=—Characters _acquired_ by the soma (i. e. variations acquired after birth by the action of the environment), as distinguished from characters _produced_ and potentially present from the first by a union of two masses of germ-plasm—plasmogenetic characters (q. v.). =Somatoplasma-plasm.=—_See_ p. 32. =Specialized congenital characters.=—_See_ p. 65. =Spermathecae.=—Organs for the storing of the seminal fluid received in copulation. =Spermatogenesis.=—The precise development of spermatozoa. =Spermatozoa.=—The essential elements in the male seminal fluid, and secreted by the testis—the male reproductive gland. =Stirp= (=Gallon=).—_See_ p. 58. =Telegony.=—_See_ p. 141. =Unicellular Organisms.=—Organisms composed of a single cell only, as distinguished from those consisting of aggregations of cells—Multicellular organisms. =Vertebrata.=—Animals with a backbone and a ventral heart.