FOR this lecture in memory of George Morrison Coates, it is fitting to choose a subject that is in the forefront of otosurgical interest. It is easy to forget that in the beginning the operation that revolutionized otologic surgery, the one-stage fenestration operation, was vigorously opposed by nearly all of the leaders in otology. Dr. Coates and Dr. James Babbitt of Philadelphia, almost alone of the senior otologists of that time, recognized this new procedure as a significant advance and gave their full support to the small group of Lempert's early students who met once a year as the Otosclerosis Study Groups. Until his death, Dr. Coates attended each meeting of this group, lending his mature judgment to the discussions. For this lecture, I have chosen to discuss the endolymphatic sac, a neglected part of the auditory mechanism, nearly forgotten by the clinician and ignored by the otologic surgeon. The