370 B. F. KINGSBURY 



above removes many of the difficulties that beset the inter- 

 pretation of the thymus as a branchiomeric organ definitely 

 located in the branchial epithelium. Recognizing that the 

 thymus-forming factors are not intrinsic but extrinsic, i.e., partly 

 a function of position and relation, it is no longer necessary 

 directly or completely to homologize thymus bodies in different 

 forms, since it is obvious that different growth conditions may 

 determine the thymus development from quite different por- 

 tions of the branchial epithelium, and portions that in one 

 form may persist and undergo thjonus transformation, in others 

 may degenerate and disappear without the characteristic re- 

 action appearing. It is only for convenience, and in view of 

 the final result, that the purelj^ epithelial stage may be termed 

 ^thymus' or 'thymus anlage.' 



Turning now to the question of a 'thymus IV anlage:' In 

 the human embryo it is clear that there is no reason for serially 

 homologizing the ventral diverticulum IV with the thymus III. 

 The thymus IV is variable (as indeed is thymus III) and in- 

 constant. It has never been shown that it is distinctively or 

 exclusively derived from this portion of the branchial epithelium. 

 The ventral pocket is clearly but an expression of the early and 

 temporary morphological growth relations and speedily dis- 

 appears as such (Kingsbury '14 b). It is altogether probable 

 that the thymus transformation may befall any portion of the 

 epithehum of the Complex IV (caudal pharyngeal complex) 

 which may persist in the mesenchyme. 



It may be said in closing this consideration of the thymus 

 that at most it may be stated that there is a wide-spread tendency 

 to thymus-formation in the branchial region, characterized by 

 a persistence and growth of epithelium with a characteristic 

 (though not peculiar) reaction with the adjacent tissues, under 

 conditions that are not yet fully analyzable. What these condi- 

 tions are and what determines the development of a thymus 

 or thymus bodies is unknown, and any attempt to determine 

 them awaits further analysis of the growth conditions of the 

 region, particularly in lower forms. I desire to call attention 



