Textbook of normal histology: including an account of the development of the tissues and of the organs . usstructures being comparatively late contri-Fig. 404. butions from the mesoderm. The internal ear first appears as a thick-ening and soon after depression of theectoderm within a small area on eitherside of the cephalic end of the neural tube ata level corresponding to about the middleof the future medulla. This auditory pitis widely open for a considerable time anddistinguished by the great thickness of itsdepressed wall, which contrasts stronglywith the adjacent ectoderm. After a timethe
Textbook of normal histology: including an account of the development of the tissues and of the organs . usstructures being comparatively late contri-Fig. 404. butions from the mesoderm. The internal ear first appears as a thick-ening and soon after depression of theectoderm within a small area on eitherside of the cephalic end of the neural tube ata level corresponding to about the middleof the future medulla. This auditory pitis widely open for a considerable time anddistinguished by the great thickness of itsdepressed wall, which contrasts stronglywith the adjacent ectoderm. After a timethe lips of the pit approximate until bytheir final union the cup-like depressionis converted into a closed sac, the oticvesicle. The otic vesicle, after severing all con-nection with the ectoderm, gradually re-cedes from the surface in consequence ofthe growth of the intervening mesodermiclayer ; it next loses its spheroidal form and becomes pear-shaped,with the smaller end directeddorsally. This diverticulumis the first appearance of therecessus vestibuli, a divi-sion of the embryonal laby- Fig. Section through developing earof nine-and-a-half-day rabbit em-bryo : e, ectoderm thickened andinvaginated to form auditory pitato; in, surrounding still undif-ferentiated mesoderm; n, liningof neural tube ; v, blood-vessel.
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Keywords: ., bookauthorpiersolgeorgeageorgea, bookcentury1800, bookdecade1890
