central nervous system. The fibers have cross 

 lines. 



structure: construction and arrangement of parts 

 or organs of an organism. 



style: usually slender portion found in many pis- 

 tils between stigma and ovary. 



sulfa drugs: large variety of compounds (sulfanil- 

 amide, etc.) in common use since the 1930's 

 in treating various diseases and infections. 



survival of the fittest: according to Darwin's the- 

 ory of natural selection the survival of the 

 best adapted organisms, which thus become 

 the parents of the next generation. 



symbiosis (sim-by-oh'sis) : intimate living to- 

 gether of two species of organisms to the 

 advantage of each, as the alga and fungus in 

 a lichen. 



synapse (sin'aps) : region where a nerve impulse 

 is transferred from one neuron to another; 

 includes the end brush of one cell and the 

 dendrites of another. 



tadpole: water-living form in the life history of 

 frogs, toads, etc. ; it hatches from the egg. 



tagged atoms: also called tracers; radioactive iso- 

 topes which, when introduced into the body, 

 can be traced and thus help in explaining the 

 use of various elements in body processes. 



taproot: long, main root of a plant, corresponding 

 to a main stem. 



tendril (ten'dril) : thin, stemlike part of many 

 climbing plants, responsive to contact and 

 twining around any other object. 



tentacles (ten'ta-culs) : slender, flexible projec- 

 tions of an animal; hydra and jellyfish use 

 them for food getting and protection. 



testis (tes'tis) ; pi. testes: another name for the 

 spermary of a male animal. 



thallophytes (tharoh-fites) : members of a phylum 

 of simple plants without true stem, root, or 

 leaf, such as algae, fungi, and lichens. 



thiamin (thigh'a-min) : one of the vitamins of the 

 B complex group (Bj) ; needed in the diet to 

 prevent beriberi. 



thoracic (thor-as'sick) duct: large lymph-carrying 

 vessel that empties lymph into a vein under 

 the left collarbone. 



thorax (thor'aks) : in higher vertebrates, the part 

 of the body between the neck and abdomen, 

 containing heart and lungs; in insects, the 

 part between the head and abdomen. 



thoroughbred (thor'oh-bred) : animal bred from 

 parents with certain desirable traits and 

 whose pedigrees are registered. 



thymus (thigh'mus) : organ in man lying behind 

 the upper part of the breastbone; plays an 

 important role in children; may be a duct- 

 less gland. 



thyroid (thigh'royd) gland: ductless gland in the 

 neck just below the voice box; secretes thy- 

 roxin. 



thyroxin (thigh-rox'in) : hormone secreted by the 

 thyroid gland, containing comparatively large 



amounts of iodine. It speeds up oxidation in 

 the body. 



tissue (tish'you) : group of cells, sometimes in- 

 cluding cell products, which are alike in 

 structure and do the same kind of work. 



toxin (toks'in) : specific poison made by patho- 

 genic microorganisms and causing a specific 

 disease; also, poisons produced by certain 

 animals and plants. 



toxoid (toks'oid) : weakened toxin used for in- 

 oculation into people and animals against 

 diphtheria, tetanus, etc. 



trachea (tray'kee-a) : in higher vertebrates the 

 windpipe; in insects one of the tubes that 

 branch throughout the body carrying air. 



transpiration (trans-pir-ay'shun) : diffusion into 

 the surrounding air of water from the cells 

 of a plant leaf. 



trichinosis (trick-i-noh'sis) : disease of men, pigs, 

 and some other animals caused by the tri- 

 china worm which enters the digestive tract 

 and settles in the muscles. 



trilobite (try'lo-bite) : any of a group of extinct 

 arthropods that lived in great abundance in 

 the seas during the Paleozoic era. 



tropism (troh'pism) : the turning of a plant or 

 animal or one of its parts toward (positive 

 tropism) or away from (negative tropism) 

 the source of a stimulus. 



trypsin (trip'sin) : digestive enzyme in pancreatic 

 juice that breaks down proteins and peptones 

 into still smaller molecules. 



tuber (too'ber) : fleshy underground stem capable 

 of producing new plants by vegetative repro- 

 duction, as the Irish potato. 



twins: see fraternal twins and identical twins. 



ultraviolet: light rays invisible to us, lying just 

 beyond the violet end of the spectrum. 



umbilical (um-bil'ick-el) cord: in a mammal the 

 cord that connects the embryo with the pla- 

 centa; blood vessels from the placenta to the 

 embryo lie in it. 



unit characters: see independent assortment. 



urea (you-ree'a) : nitrogen compound produced 

 and given off as a waste in animals when pro- 

 teins are oxidized or when amino acids are 

 broken down in the liver. 



ureter (you-ree'ter) : in vertebrates a tube lead- 

 ing from the kidney to the bladder. 



"use and disuse": name given to Lamarck's theory 

 of how organisms change through the ages. 



uterus (you'ter-us) : in mammals the organ in 

 which the embryo lies during its develop- 

 ment. 



vaccination (vax-i-nay'shun) : process of making 

 a person immune to smallpox; now applied 

 to active immunization against other dis- 

 eases, such as typhoid. 



vaccine (vax'seen) : virus of cowpox, used for 

 preventing smallpox; nowadays any modified 

 disease-producing material used to prevent a 

 specific disease. 



597 



