It occasionally happens that in extracting permanent teeth a small fibrous bag is found at the apex of the root, often no larger than an apple seed, though sometimes it may be as large as a bantam's egg, filled with fluid, and often containing crystals of cholesterin. These sacs, or dental cysts, occur in connection with the dead roots of mandibular and maxillary teeth, especially molars and premolars. They sometimes attain a considerable size in the upper jaw when they invade the antrum, and some of these cysts are sufficiently large to simulate an abscess of that cavity. Dental cysts are often bilateral and occasionally multiple. The constant association of these cysts with the dead roots of permanent teeth has led many observers to regard them as pus-sacs with thick fibrous walls. Others, having demonstrated the existence of an epithelial lining in many of these cysts, believe that they arise in embryonal "rests," known as "paradental epithelial remnants".