DEVELOPMENT OF HUMAN PHARYNX 365 



the point of view it represents will be apparent from even the 

 scant consideration that follows. 



If the thymus represents a form of 'reaction of degeneration' 

 as outlined above, other instances of similar nature should occur, 

 as indeed is the case. Jolly particularly has created a group 

 of "lympho-epithelial organs" in which he has grouped Peyer's 

 patches, tonsils, esophageal lymphoid organs of birds, the 

 follicles of the bursa Fabricii of birds of prey, the lymphoid 

 papillae of anal glands, placoid thymus of teleosts, bursa Fabricii 

 (of other birds), and the thymus of most vertebrates. To these 

 might be added a number of others, such as the amphibian 

 tonsils (Kingsbury '12), Kiemenreste of Anura, the tubal tonsils 

 of birds, and others will doubtless be added as the field of in- 

 vestigation is extended. In all these cases there is illustrated 

 a more or less intimate reaction of epithelium and lymphoid 

 cells (lymphocytes) . In view of what seems to the writer strong 

 and convincing evidence of the mesenchymal origin of such 

 lymphatic cells (lymphocytes) and with the acceptance of this 

 origin, these structures illustrate a reaction of epithelium and 

 mesenchyme (connective tissue) characterized by free prolifera- 

 tion of the cells of the latter with an invasion and infiltration of 

 the former. An essentially regressive character of the epithelial 

 structure in many of the instances is not clearly apparent, and 

 awaits more light on the biological significance of the part, as 

 in the case of the bursa Fabricii of birds. In other cases the 

 degenerative element is clear, as in the case of glands (e.g., 

 tubal tonsils of birds) where the lymphatic cells infiltrate and 

 replace the gland, entirely or in part. 



Stohr in 1891, in dealing with the development of the palatine 

 tonsils, concluded with a general discussion containing the 

 following pertinent paragraph (translation) : 



It is probable that most of those leucocytes that occur outside the 

 lymphatic nodes (lymph glands) and outside of lymph and blood ves- 

 sels are the agents of resorption processes whether the same occur in 

 the service of digestive processes or are for the purpose of removing 

 bodily structures which have entered upon a partial or complete re- 

 gression. As proof of the latter,. one may point to the frequent occur- 



