DEVELOPMENT OF HUMAN PHARYNX 363 



consideration to the mammalian thymus and the third pouch — 

 the entire branchial entoderm and that portion of the adjacent 

 ectoderm which becomes cervical vesicle. 



It is necessary, therefore, to find some basis for understanding 

 the development of the thymus other than the assumption that 

 it is a representative of hranchiomeric organs whose anlages are 

 definitely located in one or more of the branchial pouches and 

 potentially in all. Such a basis for its interpretation is found 

 in the recognition that it is a structure whose appearance is 

 determined by extrinsic factors of relation and position and not 

 intrinsic factors located in any particular group of cells. In 

 support of such an interpretation and giving us, I believe, a 

 better comprehension of its morphologic significance, we have the 

 fundamental plan of its histogenesis. 



The histogenesis of the thymus, it is true, has been a mooted 

 question, there being six different interpretations in two rather 

 natural groups. The results of recent workers, including 

 the extensive work of Maximow ('09-) and Hammar ('07-) 

 and his pupils, supplented by that of Pappenheimer ('13), 

 Badertscher ('15 b), Hartmann ('14), and others (e.g., Jolly, 

 '11, Hart '12) show quite conclusively, I think, that the thymus 

 is formed by extensive infiltration of the epithelium by cells 

 from outside with the characteristics and potentialities of lym- 

 phocytes, which there proliferate. The*se cells are derived, 

 according to Maximow, Badertscher and Hartmann, from the 

 connective tissue (mesenchyme) while Hammar ('11) stiU recog- 

 nizes the possibility or probability that they come from the 

 blood. I believe that the correct interpretation is that of Maxi- 

 mow, Hartmann and Badertscher — -the work of the last named 

 having been closely followed by me. Briefly stated, and in 

 somewhat general terms and with the attempt to emphasize 

 the fundamental in the process: The epithelium of the third 

 pouch complex due to its peculiar location, by growth and the 

 attending growth shiftings, as has been described above, becomes 

 transformed into an elongated epithelial tube. The caudal 

 portion which becomes the thymus lobe (in man) grows exten- 

 sively (fig. 24) but in a manner not in the least typical of an epithe- 



