360 B. F. KINGSBUEY 



and evolution follow inevitably in the growth of the body from 

 intrinsic causes not analyzable in the present stage of our knowl- 

 edge. As soon as it is recognized that the existence of these 

 organs is itself a subject of scrutiny and analysis, the determina- 

 tion of the developmental conditions and growth transformations 

 becomes a matter of considerable importance, and — not to mini- 

 mize in the least the ultimate importance of other sides of the 

 problem — a correct interpretation of the morphogenesis and 

 histogenesis remains a fundamental prerequisite for the ade- 

 quate interpretation of the significance of these structures as 

 bodily organs. 



Thymus 



In the case of the thymus, the prevailing tendency at the 

 present time is to view it as an 'epithelial organ' the factors 

 for whose appearance are intrinsic in certain cell groups in the 

 third branchial pouch, where, in the ventral diverticulum the 

 thymus anlage as early as 5 or 6 mm. embryos is indicated by 

 the thicker character of the epithelium that by its growth pro- 

 duces the thymus body or gland. So universally have recent 

 investigators supported this localization that it is necessary 

 to cite specific references or quote passages; Verdun ('98), Mau- 

 rer ('06), and Grosser ('11 b) have crystallized the common 

 interpretation in their comprehensive articles on the pharyngeal 

 derivatives. Localization of a thymus anlage in the ventral 

 diverticulum of the third pouch, under the sway of the M3ranchio- 

 meric organs' conception leads naturally to the homologization 

 of the comparable pockets in the second and fourth pouches 

 (fig. 12-13) as thymus bodies II and IV. Thus Groschuff ('96) 

 and Tandler ('09) term the ventral pocket of the fourth complex 

 a 'thymus IV,' and Grunwald ('10) derives the palatine tonsil 

 from the ventral pocket of the second pouch and regards it there- 

 fore as a thymus metamer. The fact that thymus bodies are 

 unquestionably developed from the Complex IV, in man occasion- 

 ally, and in certain mammals (e.g., cat) more constantly, is re- 

 garded as supporting the homology, especially since thymus 

 bodies are developed in connection with a number of branchial 



