Between 1900 and 1930, in this country and abroad, workers like Carrel and Ullmann were performing this type of experiment. Kidneys were most often used because of their simple vascular supply. The ureters functioned within minutes and gave an index to work with. Although Carrel recognized the transferred kidneys functioned normally, put out urine and maintained life, he was unaware what was causing their loss. The concept of rejection was not yet understood.