1. The acoustic ganglion arises from placodal cells which detach from the otic vesicle between the 6 ·0-mm. and 14·0-mm. stages. The accepted view that it develops from a common ‘acustico-facial’ neural crest is rejected.

  2. The initial rudiment of the acoustic ganglion is a mass of detached placodal cells which lies at the base of the vesicle and is distinctly separated from the facial crest by a zone of mesenchyme.

  3. After the 8·0-mm. stage the detachment and migration of placodal cells becomes more vigorous and the enlarging acoustic ganglion becomes secondarily apposed to the geniculate ganglion. Nevertheless, these ganglia retain their separate identity through cytological differences which rest on the fact that the young neuroblasts are already present in the geniculate ganglion before a distinct acoustic rudiment is evident. This contrast wanes gradually after the 11·0-mm. stage when the placodal cells begin to differentiate into acoustic neuroblasts.

  4. It is concluded that the acoustic ganglion arises exclusively from placodal cells and the participation of facial crest cells in the root portion, if it occurs at all, is probably negligible.

  5. The definitive vestibular ganglion is derived from the pars superior together with the lateral zone of the pars inferior in which the earliest neuroblasts appear.

  6. The definitive cochlear ganglion begins to develop rather later and arises from the remaining medial zone of the pars inferior with the addition of further diffuse placodal contributions from the base of the vesicle.

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