The palatine tonsil as an evolutionary novelty.

The development of the pharyngeal pouches was studied in early human embryos. A vesicular thymus primordium, separated from the lining epithelium of the second pouch, is described in 5-week-old embryo's (9 mm stage). This transitory structure disappeared in the next stage (10 mm). Much later--in the 9th week, the first branched and solid crypt developed in the same region, from the same epithelium, and in a similar way to a gland. This formation then stimulated the lymphocytic infiltration of the connective tissue around the crypt branches in the 13th week. Attention is drawn to the similarity between the closed relation of the salivary glands and the simplest tonsils in lower mammals. These findings prove the original potency of the second pouch to form a thymus as in lower vertebrates, and to form salivary glands as in birds and even lower mammals. They also prove that all these phyletic changes have been imprinted in the human ontogeny, and explain the mutual relations between the palatine tonsil and the thymus in postnatal life.