538 THE LAWS OF HEREDITY 



UMBILICAL cord ; the structure which passes from the navel of the child to the placenta. 

 UNICELLULAR ; see cell. 

 UTERUS ; the womb. 



VARIABILITY ; the tendency to vary. 



VARIATION ; a filial departure, founded on a germinal change, from the parental type. 



WASTE-PRODUCTS ; the final products of tissue metabolism. They are excreted by living 

 beings, and are comparable to the ash, carbonic acid, water, etc., which result from the 

 combustion of coal. 



ZYGOTE ; the fertilized ovum. 



