612 



of the lungs, it is probable that at least two branchial pouches 

 intervened in the ancestors of the mammals between the fourth pouch 

 and the lungs. The acceptance of a sixth potential pouch carries 

 with it a branchial arch and blood vessels which have disappeared 

 witli it more or less completely. It thus becomes a question whether 

 the pulmonic arch of mammals should not more correctly be num- 

 bered VII instead of VI if numbered at all. To accept Rabl's homol- 

 ogization of the ultimobranchial body with a sixth pouch at once 

 raises the question as to whether Tandler's fifth aortic arch is really 

 V or VI. The conservative attitude of F. T. Lewis ('06), therefore, 

 in urging caution in the interpretation of the irregular bloodvessels 

 that appear in development between the fourth aortic arch and the 

 pulmonic arch seems to be quite justifiable. 



The double assumption of this pharyngeal derivative as an 

 ' ultimobranchial ' body and as a vestigial gland representing an 

 ancestral organ, clearly contains within itself an irreconcilable in- 

 consistency : both cannot be true on any morphological basis of 

 homology. 



In a study of the development of the human pharynx^), con- 

 siderable interest was felt in the developmental changes in this portion 

 of the embryonic pharynx, and it soon became evident that it merited 

 a somewhat special study. The number of series available was so 



1) This investigation was carried out in the Department of Comparative 

 Anatomy of the Harvard Medical School, and is based upon the collection 

 of human embryos generously placed at my disposal by Professor Minot, 

 supplemented by the series of human embryos at Cornell University, par- 

 ticularly those loaned me by Professor S. H. Gage. I desire to express my 

 appreciation of the cordial and generous help of all who have cooperated 

 with me in this study. To Professor C. S. Minot particularly am I indebted 

 for helpful suggestions. The embryos examined, are entered below bj^ size 

 and will be so referred to in the text. Models of tlie pharynx were prepared 

 in the case of embrj-os 3. mm., 5. mm., 7.5 mm., 9.4 mm., 10. mm., 11. mm., 



14.5 mm., 18.2 mm. The pharynx of the 4. mm., embryo had been modelled 

 by Dr. Bremer. 



Embryos examined : — 3. mm., 4. mm., 5. mm., 6.25 mm., 7.5 mm., 8.3 mm., 

 9.2 mm., 8. mm., 9.4 mm., 9.6 mm., 10. mm., 10. mm., 10.2 mm., 11.0 mm. 

 (N.B.; Cr. — B. = 13. mm.), 11.5 mm., 12. mm. (1), 12. mm. (2), 13.5 mm, 



13.6 mm., 14.5 mm., 15. mm. (1), 15. mm. (2), 16. mm (1), 16. mm (2), 16.4 mm., 

 18.2 mm., 19.0 mm. (1), 19.0 mm. (2), 19.0 mm. (3), 19,3 mm., 19.7 mm., 21. mm., 

 22. mm., 22.8 mm., 23. mm. (1), 23. mm (2), 25. mm., 25.6 mm., 28.8 mm., 

 29. mm., 30. mm., 31. mm (1), 31. mm. (2), 35. mm., 36. mm., 37. mm., 40. mm., 

 41. mm., 44. mm. (1), 44. mm. (2), 44.3 mm., 48. mm. 



