The organogenesis of the membranous labyrinth of Polypterus senegalus is described. 1. The otocyst seems to be formed by the invagination of a thick portion of the deeper layer of the epidermis. 2. The 3 semi-circular canals are formed by 3 pairs of invaginations of the wall of the otocyst and at the expense of its volume; the anterior vertical canal is completed first, followed by the horizontal and then the posterior vertical one; the ampullae, not very developed, appear a long time after the formation of their corresponding canals: that of the horizontal canal appears first, followed by that of the posterior and finally that of the anterior one. 3. The sensory areas derive from a common macula which subsequently divides into 2 zones, the anterior one giving rise to the utricular macula and the anterior and horizontal cristae, the posterior one giving rise to the posterior crista and the saccular macula; from the latter subsequently develops the lagenar macula. 4. The otoconiae appear as soon as the otocyst forms; the otoliths are agglomerations of otoconias brought together by an interstitial cement. 5. The endolymphatic primordium is formed before that of the semi-circular canals; the endolymphatic sack becomes voluminous and spreads over the brain as far as the sagittal plane.