umbilical cord The structure that connects the placenta and the embryo; contains the umbilical arteries and the umbilical vein.

unicellular Single-celled.

uniformitarianism The idea that geological processes have remained uniform over time and that slight changes over long periods can have large-scale consequences; proposed by James Hutton in 1795 and reÞned by Charles Lyell during the 1800s.The principle on which modern geology was founded: processes operating today on the earth operated in much the same way in the geologic past. Sometimes expressed as "the present is the key to the past".

uninucleate Term applied to cells having only a single nucleus.

unsaturated fat A triglyceride with double coavent bonds between some carbon atoms.

uracil The pyrimidine that replaces thymine in RNA molecules and nucleotides.

ureter A muscular tube that transports urine by peristaltic contractions from the kidney to the bladder.

urine Fluid containing various wastes that is produced in the kidney and excreted from the bladder.

uterus The organ that houses and nourishes the developing embryo and fetus. The womb. Female reproductive organ in which the fertilized egg implants.

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