Nephric tubules (nephrons) form embryonically along the nephric ridge. The ridge comprises three regions; anterior, middle and posterior. The functional kidney uses nephrons from only one or two regions of the ridge. A pronephric kidney uses only the nephrons from the anterior region of the ridge. A mesonephric kidney uses only the middle region, a metanephric kidney uses only the posterior region and an opisthonephric kidney uses the middle and posterior regions. [ http://cwx.prenhall.com/bookbind/pubbooks/martini10/chapter27/custom2/deluxe-content.html ]