. A laboratory manual and text-book of embryology. Embryology. DEVELOPMENT OF THE SENSE ORGANS 383 WW/of myetencephaton cndotymptt duct Vestibular, duct corresponds to that of selachian fishes, which remains open to the exterior. In man, its dorsal extremity is closed and dilated to form the endolymphatic sac (Fig. 366 e). The differentiation of the auditory vesicle has been described by His, Jr. and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em- bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con- stituting the anlage of the cochlear duct (F
. A laboratory manual and text-book of embryology. Embryology. DEVELOPMENT OF THE SENSE ORGANS 383 WW/of myetencephaton cndotymptt duct Vestibular, duct corresponds to that of selachian fishes, which remains open to the exterior. In man, its dorsal extremity is closed and dilated to form the endolymphatic sac (Fig. 366 e). The differentiation of the auditory vesicle has been described by His, Jr. and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em- bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con- stituting the anlage of the cochlear duct (Fig. 366 a). The wider dorsal portion of the otocyst is the vestibular anlage and it shows indications dorsally of the de- veloping semicircular canals. These are formed in n mm. embryos as two pouches, the anterior and posterior canals from a single pouch at the dorsal border of the otocyst, the external canal later from a lateral outpocketing (Fig. 366 d). The mar- gins of these pouches are thickened, but elsewhere their walls are flattened together and fused to form an epi- thelial plate. Three such epithelial plates are produced and internally about the periphery of each plate canals are left communicating with the cavity of the vestibule. Soon the epithelial plates are resorbed, leaving spaces between the semicircular epi- thelial canals and the vestibule (Fig. 366 c). Dorsally a notch separates the anterior and posterior canals. Of these the anterior is completed first, next the posterior canal. The external canal is the last to develop. In a 20 mm. embryo (Fig. 366 e) the three canals are present and the coch- lear duct has begun to coil like a snail shell. It will be seen that the anterior and posterior canals have a common opening dorsally into the vestibule, while their opposite ends and the cranial end of the external canal are dilated to form ampulla. In each ampulla is located an end organ, the crista acustica, which will be referred to later. By a constriction of i
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectembryology, bookyear1
