456 ELEMENTS OF BIOLOGY 



Pleural cavity. A cavity containing the lung. 



Plica. A transverse fold in the mammaUan intestine. 



PoiKiLOTHERMAL (Adjective), An animal whose body temperature varies 



with that of the environment. 

 Pollen. The microspores produced in the anthers of the flower. They 



are the spores which when transferred to the stigma develop into 



the sperm-producing gametophyte generation. 

 PoLocYTE. A non-functional cell produced during the maturation of 



the ovum. When the primary oocyte divides, one of the two daugh- 

 ter cells is the first polocyte. When the secondary oocyte divides, 



one of the products is the second polocyte. 

 PoLYHYBRiD. The progcny of a cross between two parents that differ in 



many pairs of alternative characters. 

 Potential energy. Energy in a state of suspended activity. 

 Preformation. A concept of embryonic development which holds that 



the process is one of unfolding of structures already present in the 



germ. 

 Presbyopia. Far-sightedness. 

 Primary oocyte. An early stage in oogenesis during which the nuclear 



phenomena of synapsis occur. 

 Primary spermatocyte. An early stage in spermatogenesis during 



which the nuclear phenomena of synapsis occur. 

 Primordium. a structure in an embryo destined to develop Into a specific 



organ or part of the adult. 

 Proboscis. A snout that may consist of mouth parts as in some Insects, 



or of other head parts, for example, the trunk of the elephant. 

 Proglottid. A segment of the tapeworm containing the reproductive 



and excretory organs and identical with adjoining segments. 

 Pronephros. The primitive excretory organ of vertebrates. 

 Prophase. An arbitrary stage in the process of mitotic cell division char- 

 acterized by the formation of a spireme of chromatin, the disapr 



pearance of the nuclear membrane, and the formation of the 



mitotic spindle. 

 Protein. A complex organic molecule characterized by containing 



COOH and NHo radicals. 

 Proteolytic enzyme. An enzyme that digests proteins. 

 Proteose. An intermediate product, the first step in the digestion of 



proteins. 



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