. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE BRANCHIAL EPITHELIAL BODIES 295 This bursa has very generally been regarded as the persistent remains of Rathke's pouch (p. 285), especially since it is much more pronounced in fetal than in adult life. It has been shown, however, that it is formed quite independently of and posterior to the true Rathke's pouch (Killian), though what its significance may be is still uncertain. The tonsils are formed from the epithelium of the second bran- chial groove. At about the fourth month solid buds
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE BRANCHIAL EPITHELIAL BODIES 295 This bursa has very generally been regarded as the persistent remains of Rathke's pouch (p. 285), especially since it is much more pronounced in fetal than in adult life. It has been shown, however, that it is formed quite independently of and posterior to the true Rathke's pouch (Killian), though what its significance may be is still uncertain. The tonsils are formed from the epithelium of the second bran- chial groove. At about the fourth month solid buds begin to grow from the epithelium into the subjacent mesenchyme, and depressions appear on the surface of this region. Later the buds become hollow by a cornification of their central cells, and open upon the floor of the depressions which represent the crypts of the tonsil. In the meantime lymphocytes, concerning whose origin there is a difference of opinion, collect in the subjacent mesenchyme and eventu- ally aggregate to form lymphatic follicles in close relation with the buds. Whether the lymphocytes wander out from the blood into the mesenchyme or are derived directly from the epithelium or the mes- enchyme cells is the question at issue. The tonsil may grow to a size sufficient to fill up completely the groove in which it forms, but not infrequently a marked depression, the fossa supratonsillaris, exists above it and represents a portion of the original second branchial furrow. The groove of Rosenmuller, which was at one time thought to be also a remnant of the second furrow, is a secondary depression which appears in embryos of cm. behind the opening of the Eustachian tube, in about the region of the third branchial furrow. The Development of the Branchial Epithelial Bodies.—These are structures which arise either as thickenings or as outpouchings of the epithelium lining the lower portions of the inner branchial fur- rows. Five pairs of these structures are dev
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