Experimental Demonstration of Tongue Muscle Origin in Chick Embryos

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ABSTRACT Since in all classes of vertebrates the tongue muscles are innervated by nerve XII, a segmental nerve of the occipital region, it is usually argued on this criterion alone that they originate from occipital myotome tissue. Descriptive evidence in support of this generalization is, however, far from adequate. The most complete accounts that exist refer to one amphibian and two reptile species. In the amphibian Necturus, Platt (1897) observed that ventral outgrowths of the 3rd and 4th occipital myotomes became tongue muscles, and Edgeworth (1935) has described the development of tongue muscles in the reptiles Sphenodon and Lacerta, from ventral parts of two occipital and two cervical myotomes, all innervated by nerve XII. In avian and mammalian embryos, however, early muscle rudiments are extremely difficult to recognize with any certainty histologically. Although Butcher (1929) and Bates (1948) have observed ventral outgrowths of occipital myotomes in rat and cat embryos, and Bates claimed that these are recognizable as tongue muscle primordia in later stages, both these authors drew their conclusions from relatively few fixed specimens, and in Bates’s photographs the rudiments are not nearly as distinct as his diagrams suggest they are. The same criticism applies to Hunter’s description (1935) of occipital myotome rudiments that apparently develop into tongue muscles in the chick. The only other detailed description of tongue muscle development in birds is that of Kallius (1905) who dealt with advanced stages when the muscle primordia are already in situ.