GLOSSARY 405 



Unconformity. A sharp contrast, often a lack of parallelism, between adjoining rock 

 strata, cansod by a long period of erosion. 



Uniformitarianism. The doctrine that geological processes of the past were similar 

 to those of the present time. 



Unisexual. Involving but one sex, the female; applied to parthenogenetic reproduc- 

 tion. 



Unit character. A hereditary trait that behaves as a unit in transmission, being 

 capable of inheritance independently of other luiit characters. 



Universal symmetry. An arrangement of the parts of an object or organism such 

 that it is capalsle of being divided into symmetrical halves by an infinite number 

 of planes passing in any direction through a central point. 



Urea (u re' a). A substance, C0(NH-))2, produced by the decomposition of proteins 

 and some other substance in organisms. 



Ureter (it re' ter). A tube conducting urine awaj' from the kidney. 



Urethra {u re' thra). The duct by wliich urine is discharged from the bladder. 



Urinary bladder. A bag in which urine is stored. 



Urine. The liquid waste excreted bj^ kidneys. 



Uriniferous tubule. One of the many coiled tubes making up the bulk of the kidney 

 in vertebrates. 



Urinogenital system. A group of organs concerned with both excretion and reproduc- 

 tion in vertebrates. 



Uterus («' te rus). A modified portion of the oviduct in which the eggs undergo at 

 least part of their development. Strictly the term uterus is apphcable only in 

 animals in which the developing embryo becomes attached to the wall of the 

 organ. 



Vacuole. A region within a cell occupied by a liquid other than protoplasm, usually 

 water with various substances in solution. 



vVagina. The passage leading from the uterus to the exterior in many animals. 



Valence. A measure of the mmiber of other elements or radicals with which a given 

 element or radical may combine; it is determined by the number of electrons in 

 the outer layer. 



Variety. In taxonomy, a division of a species; a group of individuals within a species 

 tliat differ in some minor respect from the rest of the species. 



Vascular tissue. Blood or lymph, or the more liquid parts of blood-producing organs. 



Vas deferens (vas' def er enz) (pi., vasa deferentia, vas' a def er en' shi a). A duct 

 conveying spermatozoa from the testis to the exterior. 



Vas efferens {vas'' ef fer enz) {pi., vasa efferentia, vas' a ef fer en' shi a). One of a 

 number of minute tubes leading away from a testis, serving to convey the sper- 

 matozoa. They lead into a larger tube called in many cases the vas deferens. 



Vaucheria {vaw ke' ri a). A multinucleate fresh-water alga. 



Vegetative. Concerned with nutrition. When applied to an egg, meaning that side 

 near wliich the yolk is accumulated (vegetative pole). 



Vein. A vessel conveying toward the heart blood which has already traversed capil- 

 laries since leaving the heart. 



Ventral. Literally, pertaining to the belly; hence, usually, lower. 



Ventricle. The posterior chamber of the heart in fishes, amphibia, and some reptiles, 

 and one of the two posterior chambers in higher vertebrates. Its function is the 

 propulsion of the blood through the main arteries and connecting vessels. 



Vermiform appendix. A narrow blind pouch forming a prolongation of the caecum. 



Vertebrata. A subphylum of the phylum Chordata, comprising the backboned 

 animals. For definition see Chap. 19. 



