The pronephros appears early in the fourth week as segmentally arranged cell (nephrotomes) and tubular (pronephric tubuli) structures in the cervical region of the intermediate mesoderm. The pronephric tubules open into the intraembryonic coelom (coelomic cavity), while the pronephric duct runs caudally and opens into the cloaca. By the beginning of the 5th week, the regression of the vestigial nephrotomes is complete.