GLOSSARY 507 



Plasma. Liquid portion of the blood, lymph, and tissue fluid. 



Plasma-membrane. Living cell membrane, as distinguished from the 

 cell wall which may also be present. 



Plastid. A specialized cytoplasmic body. See Chloroplast. 



Plexus. The intermingling of fibers from one nerve with those of an- 

 other to form a network; e.g., sciatic plexus. 



Pollen. The microspores of Seed Plants. 



Pollination. The transference of pollen to the stigma of the pistil in 

 Seed Plants. Eventuates in fertilization. 



Polocytes. Tiny abortive cells arising by division from the egg during 

 maturation. Polar bodies. 



Polymorphism. Occurrence of several types of individuals during the 

 life history, or composing a colony; e.g., in some Hydroids. 



Polyp. Hydra, or a Hydra-like individual of Hydroids, Corals, and other 

 Coelenterates. 



Population. Entire group of individuals from which samples are taken 

 for genetical study. Usually comprises several pure lines. 



Potential Energy. Energy possessed by virtue of stresses, i.e., two 

 forces in equilibrium. Criterion is work done against any restoring 

 force; e.g., kinetic energy of sunlight through agency of chlorophyll 

 separates C0 2 into C and 2 and thereupon is represented by an equal 

 amount of chemical potential energy. Restoring force is here chemical 

 affinity. Similarly a raised weight possesses gravitational potential 

 energy in amount equal to kinetic energy expended in raising it. See 

 Kinetic Energy and Conservation of Energy. 



Preformation. The abandoned doctrine that development is essentially 

 an unfolding of an individual ready-formed in the germ. See Epigenesis. 



Pronephros. Primitive kidney of Vertebrates. 



Prophase. Preparatory changes during mitosis leading to the disposition 

 of the chromosomes in the center of the cell (equatorial plate) ready 

 for division. See Metaphase. 



Prosopyles. Pores leading into the flagellated chambers of Sponges. 



Prostate Gland. An accessory male genital gland in Mammals. 



Prostomium. A lobe which projects from the first segment of the body of 

 the Earthworm and forms an upper lip. 



Protein. A class of complex chemical molecules, containing nitrogen, 

 which form the chief characteristic constituent of protoplasm. 



Protophyta. Unicellular plants. 



Protoplasm. The physical basis of life. Matter in the living state. 



Protopodite. The basal portion of the typical Crustacean appendage 

 from which arise the endopodite and exopodite. 



Protozoa. Unicellular animals, or colonies of animal cells not differ- 

 entiated to form tissues; e.g., Amoeba, Volvox. 



