Glossary - 695 



tween all other parts of the nervous system and the 

 cerebrum. 



Thyroid gland. A ductless gland located in the 

 throat region, below the pharynx, in vertebrates. 



Thyroxin. A hormone secreted by the thyroid 

 gland. 



Tissue. An aggregate of cells of a particular type, or 

 at most a few types, with more or less intercellular 

 material of a particular type. 



Tracheids. Elongate hollow units ("dead cell walls 

 without protoplasm) , with oblique, perforated end 

 walls, in the xylem portion of vascular bundles. 



Transpiration. A controlled elimination of water 

 vapor from plants, especially through the sto- 

 mata. 



TRirLOiD. Having three haploid sets of chromosomes. 



Tropism. A movement, of growth, orientation, or 

 locomotion, whose direction is determined by the 

 direction from which the stimulus impinges on the 

 organism. 



Trypsin. An enzyme secreted by the pancreas, which 

 catalyzes the hydrolysis of proteins. 



Tuber. A short, thick, subterranean stem with nu- 

 merous buds; e.g., a potato. 



Turgor. The taut condition of a cell due to absorp- 

 tion of water to the limit of distensibility of the 

 cell wall. 



Turgor movements. Movements effected by local 

 changes of turgor. 



Ultramicrobe. An ultramicroscopic organism. 



Ultramicroscope. A microscope in which light is 

 thrown onto the object from the side, so that par- 

 ticles appear as bright specks against a dark back- 

 ground. 



Ultramicroscopic. Invisible with the ordinary micro- 

 scope, but visible with the ultramicroscope; i.e., be- 

 tween about 1 mp. and 100 m/i in diameter. 



Univalent. A single member of a pair of homol- 

 ogous chromosomes, (cf. Bivalent.) 



Urea. A simple organic compound, CO (NH.) „, the 

 chief nitrogenous waste product of mammals. 



Ureters. The tubes carrying urine from the kidneys 

 to the cloaca or to the urinary bladder. 



Urethra. The tube carrying urine from the bladder 

 to the exterior in mammals. 



Uric acid. A nitrogenous waste product, more com- 

 plex than urea. 



Urinary tuiiules. The tubules making up the kid- 

 ney, which produce the urine; nephrons. 



Urine. The fluid excreted by the kidneys: a solution 

 of various waste products. 



Uterus. An enlargement of the lower end of the 

 oviduct, in which the eggs are retained temporarily 

 or in which the embryo develops; in some mammals 

 (e.g., man) there is a single uterus formed by 

 coalescence of the two oviducts. 



Vacuole. A droplet of watery fluid within the cyto- 

 plasm, bounded by a membrane. 



Vagus nerves. The tenth pair of cranial nerves, com- 

 posed of parasympathetic fibers innervating most of 

 the internal organs. 



Variation. Differences among organisms related by 

 descent. 



Vascular bundles. Strands of tissue containing 

 woody fibers and tubular vessels, running length- 

 wise through the stems, roots, and leaf veins of all 

 tracheophytes. 



Vascular plants. Plants having vascular bundles; 

 i.e., pteropsidans, gymnosperms, and angiosperms. 



Vein. A relatively thin-walled, distensible blood ves- 

 sel, carrying blood from the capillaries back toward 

 the heart, (cf. Artery.) 



Ventral. Pertaining to the belly side of an animal. 



Ventricle. A very thick-walled chamber of the heart, 

 which receives blood from the auricle and pumps 

 it into the arteries. 



Vertebrae. The series of short bony or cartilaginous 

 segments making up the vertebral column, or back- 

 bone, of vertebrates. 



Vestigial orcan. A degenerate, useless remnant of an 

 organ. 



Viability. Ability to live. 



Villi. Small cylindrical projections of the inner sur- 

 face of the small intestine. 



Viruses. Ultramicroscopic intracellular infective 

 agents. 



Vitamins. A heterogeneous group of relatively sim- 

 ple organic food substances required by animals and 

 some other organisms in very small amounts. 



Viviparous. Giving birth to fairly well-developed 

 young, (cf. Oviparous.) 



Warm-blooded animals. Animals whose body tem- 

 perature is (normally) practically constant and usu- 

 ally higher than that of the environment; including 

 the birds and the mammals. 



X chromosomes. The sex chromosomes of which 

 there are a pair in the female but only one in the 

 male, in most dioecious diploid organisms. 



Xylem. That portion of the vascular bundles which 

 contains the ducts and tracheids, usually the inner 

 portion of the bundles; constituting the wood in 

 woody plants, (cf. Phloem.) 



Y chromosome. A chromosome in the male, which 

 pairs with the X chromosome at synapsis, and 

 which often contains few or no active genes. 



Yolk. Stored food substances in egg cells. 



Zoospores. Flagellate spores, in many algae. 



Zygospore. A thick-walled, resting zygote, in many 

 algae. 



Zygote. The cell formed by the union of two gam- 

 etes in sexual reproduction. 



