GLOSSARY 473 



Parthenogenesis. Development of an egg without fertilization. 



Partial pressure. The pressure exerted by one component of a mix- 

 ture of gases. The pressure of each component is proportional to its 

 quantity. 



Parturition. Act of bringing forth young. 



Paternal. Pertaining to, or derived from, the male parent. 



Pathogenic. Disease-producing. 



Pelagic. Said of the habitat of organisms living at or near the surface of 

 large bodies of water. 



Pelvis. The posterior region of the abdomen in vertebrates. The pelvic 

 girdle. A region in the kidney. 



Penis. An intromittent organ of the male by means of which spermatozoa 

 are transferred to the vagina of the female. 



Pericardium. In vertebrates the peritoneal sac surrounding the heart. 



Peristalsis. The rhythmic contraction of the intestinal wall. 



Peritoneum. The membrane lining the coelom of vertebrates. 



Phagocyte. A cell which ingests and destroys waste and harmful material, 

 bacteria, etc., in the body. 



Pharynx. The region of the alimentary tract between the mouth and 

 esophagus. 



Phenotype. The somatic complex of an organism or a group of organisms 

 regardless of the potential germinal possibilities. 



Photosynthesis. The synthesis, or building up, of complex organic 

 compounds from relatively simple inorganic substances through the 

 agency of sunlight in the presence of pigments like chlorophyll. A 

 natural process in plants. 



Phylogeny. The developmental history of the race. 



Phylum. A main subdivision of a plant or animal kingdom. 



Placenta. A composite maternal and fetal organ of the mammalian 

 uterus which serves to attach the embryo, to supply nourishment and 

 oxygen from the maternal blood, and to remove waste products. It is 

 shed in whole or part at birth. 



Placoid scale. Consists of a rhombic basal plate of dentine (bone) from 

 the middle of which a spine projects. In the spine is a pulp cavity with 

 blood vessels. Characteristic of Elasmobranchs. Forerunner of verte- 

 brate tooth. 



Plasma. The liquid part of the blood. Blood minus corpuscles. 



Plexus. A union of several nerves to form a network. 



Poikilothermal. Having a body temperature slightly higher than that 

 of the environment. 



Polar body. One of the minute cells formed in the maturation divisions 

 of the egg. 



Polarity. Differentiation at the two ends of an axis. 



Polymorphism. Several forms. The existence of two or more types of 

 individuals in a species, as in the honeybee. 



Polyp. A Hydralike coelenterate. 



Potential energy. The energy a body possesses by virtue of its position. 

 A lifted weight has potential energy in proportion to the kinetic energy 



