GLOSSARY 447 



PHOTOSYNTHESIS. Process by which complex compounds are built 

 up from simple elements through the energy of sunlight absorbed 

 by chlorophyll, or a functionally similar pigment. 



PHYLOGENY. The ancestral history of the race. See Ontogeny. 



PHYLUM. In classification, a main subdivision of the animal or 

 plant kingdom. See Class. 



PHYSIOLOGY. The study of the functions of animals and plants. 

 The mechanical and chemical engineering of organisms. 



PINEAL BODY. An outgrowth from the upper wall of the diencepha- 

 lon. The vestige of an additional pair of eyes possessed by the 

 ancestors of existing Vertebrates. Fcssibly functions as an en- 

 docrine gland in Mammals. Brow-spot of Frog. 



PISTIL. Organ of the flower, composed of ovule case, style, and 

 stigma. See Carpel. 



PITH. Middle part of the central cylinder of a plant shoot. Func- 

 tions largely for the storage of water and food. 



PITH RAYS. Extensions of the pith which radiate between the 

 vascular bundles to the bark. Medullary rays. 



PITUITARY BODY. An ingrowth of the ectodermal tissue above the 

 mouth and the tip of the infundibulum from the ventral wall 

 of the diencephalon unite to form a gland-like structure (pitui- 

 tary body or hypophysis) . 



PLACENTA. A Mammalian organ adapted for the interchange of all 

 nutritive, respiratory, and excretory materials between the 

 embryo (foetus) and mother. It also serves as an organ of 

 attachment. In the higher Mammals it is composed of both 

 foetal and maternal tissues. See Umbilical Cord. 



PLASMA. Liquid portion of the blood. 



PLEXUS. Intercommunication of the fibers from one nerve with 

 those of another to form a network of nerves; e.g., branchial and 

 sciatic plexus. 



POLAR BODIES. Tiny abortive cells arising, by division, from the 

 egg during maturation. 



POLE CELLS. Two cells which give rise to the mesoderm in the 

 development of the Earthworm and its allies. 



POLLEN. The microspores of Flowering Plants. 



