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In 7 mm rat embryos some sensory fibers from the four cranial nerves that innervate the mouth, pharynx and larynx have grown almost into contact with the basement membrane of the epithelium. Such fibers become more numerous but stay below it until the fetus has reached the twentieth day of gestation. At 25 mm C. R., or on the twentieth day, some fibers grow into the epithelium. Some are branched and some terminate in a Merkel's disc. By that time all types of lingual papillae can be recognized, but taste buds have not formed. Nerve endings begin to appear at about the same time in the tongue, oral surface of the palate, and oral mucosa of the cheeks and lips. Taste buds are said to appear at about the time of birth. No functional bud was found at 36 mm C. R. Intraepithelial nerve terminals have been found in the mouth at about the eighteenth day (20 mm). Other investigators have shown that rat embryos of 15.5 days can respond to the stroke of a hair across the snout by a lateral movement of the head and at 19 days can protrude the tongue independently of other movements. No record has been found concerning the age at which swallowing first occurs in the rat. It seems evident that some fibers can be stimulated at about the time they touch the basement membrane and before any recognized receptor has formed.

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