476 GENERAL ZOOLOGY 



Triploblastic. Having three germ layers. 



Umbilical cord. A ropelike connection between the mammalian embryo 



and the placenta carrying embryonic blood vessels, by means of which 



the nutritive and respiratory needs of the embryo are supplied. The 



umbilicus or navel on the abdomen of the adult marks the point of 



attachment of the cord. 

 Unguiculate. Provided with claws. 

 Ungulate. Provided with hoofs. 

 Unit character. A hereditary trait that maintains its integrity from 



generation to generation. 

 Ureter. The duct leading from the kidneys to the cloaca, or to the urinary 



bladder. 

 Urethra. The duct leading from the urinary bladder to the exterior. 

 Urine. The secretion of the kidney consisting of water and other catabolic 



products. 

 Urinogenital. Pertaining to the excretory and reproductive systems. 

 Uterus. In mammals, a specialized portion of the oviduct or oviducts in 



which the placenta forms and the embryo develops. In the frog, a 



portion of the oviduct in which eggs are held before deposition. 

 Vacuole. A small cavity or space in a cell containing fluid or other 



substances. 

 Vagina. The terminal portion of the female genital tract leading from the 



uterus or oviducts to the exterior. 

 Vascular. Pertaining to blood vessels, or blood supply. 

 Vas deferens. A duct in the male leading from the testis to the exterior. 

 Vasoconstriction. The constriction of blood vessels brought about by 



the contraction of muscles in the walls of the blood vessels. 

 Vegetal, vegetative. Nutritive. Said of the pole of the egg in which 



the yolk is concentrated. 

 Venous blood. Deoxygenated blood. 



Vertebrate. An animal with a backbone or vertebral column. 

 Viscera. Internal organs. In mammals, the contents of the abdomen. 

 Vitalism. The doctrine that living phenomena can only be explained by 



assuming the presence in protoplasm of a "vital" factor of some sort in 



addition to the factors made up of the chemical and physical properties 



of protoplasm. 

 Viviparous. Reproduction by birth. 

 Voltaic pile. A vertical series of alternating disks of two dissimilar metals, 



as zinc and copper, separated by disks of paper moistened with acid 



water. When the top and bottom are connected by a wire, a current 



of electricity is produced. 

 Zoom. One of the members of a hydroid colony. 

 Zygote. The fertilized egg, or the individual produced from the egg. 



Exconjugants are also zygotes. 



