Membranous Labyrinth.—The first anlage of the membranous labyrinth becomes visible at a very early stage. Sometimes in embryos with only two to three somites (Wilson, 1914; Ingalls, 1920; Bartelmez, 1922) a thickening of the ectoderm is seen on either side of the still open neural plate in the region which later becomes the hindbrain. This thickening becomes more prominent in the six to nine somite (2 mm.) stage; it is called the auditory placode. According to Groth's observations (1939) on the guinea pig, the placode is derived from the so-called lateral ridge (Seitenwulst) of the neural plate.In embryos with 14 to 15 somites the center of the placode sinks into the underlying mesenchyme, and a depression, the otic pit, is formed. The pit soon deepens, assumes a vesicular shape and becomes the otic vesicle, or otocyst. For a short time it remains in open communication with the
ALTMANN F. NORMAL DEVELOPMENT OF THE EAR AND ITS MECHANICS. AMA Arch Otolaryngol. 1950;52(5):725–766. doi:10.1001/archotol.1950.00700030751003
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