Endolymphatic sac specimens from human embryos, ranging in age from 8 to 20 weeks, were examined in order to get a good understanding of the embryological development. The gross development can be separated into three phases, with the first phase terminating around week 10, the second phase between weeks 11 and 15–16 and the third phase lasting to week 20. While presenting as an immature oval appendage emanating from the vestibule in the first phase, the sac then develops into its more mature shape with crypts and folds within the epithelial lining. The third period is characterized by splitting up of the lumen into separate tubules. The epithelial cells differentiate particularly during the second and third phases with light- and dark-staining cells as well as granulated cells. It is evident that the development of the endolymphatic sac is not finished by week 20, even though it has attained many of its adult characteristic