156 IVAN E. WALLIN 



plasm from the placode. The connective tissue outside the 

 placode plays the role of a reticulum insofar that the trans- 

 formation of the epithelial nuclei occur within its meshes. 



Hassal's corpuscles, or any structures comparable to them 

 have not been found in the placodes or in the connective tissue 

 outside of the placode. 



The history of the placodes in the successive developmental 

 stages indicates a gradual involution of the placodes. The 

 maximum size of the placodes occurs in a larva of 50 to CO mm. in 

 length. From this stage of development the placodes diminish in 

 size so that in the mature larva very few lymphocyte-like cells 

 remain. In the transformation of the larva, Nestler ('10) main- 

 tains that the oesophagus of the adult is formed by a transforma- 

 tion of ^'the under edge of the dorsal fold in the branchial 

 chamber" (the epipharyngeal ridge). If such a process occurs, 

 is is only after the histogenetic activities in the placodes have 

 ceased and consequently does not affect the status of an earlier 

 thymic function in these placodes. 



An examination of the descriptions given in the preceding 

 pages give the impression that the primitive thymus placodes and 

 lymphocytes are formed from an endodermal epithelium. While 

 I am not ready at this time to supply the evidence, the changes 

 which occur in the general branchial epithelium in the early 

 stages of development seem to point to a general fusion of the 

 original endoderm with the underlying mesenchyma. The 

 character of the epithelium in the more advanced larvae has such 

 an important bearing on the interpretation of the histogenesis 

 of the primitive thymus cells and lymphocytes that a separate 

 and detailed study of this process seems warranted. 



In a recent article on the Development of the Human Pharynx, 

 Kingsbury ('15) discusses the intrinsic and extrinsic factors in 

 thymus formation and challenges the view that the thymus is a 

 branchiomeric organ definitely located in the branchial epithe- 

 lium. The basis for this interpretation 



is found in the recognition that it is a. structure whose appearance is 

 determined by extrinsic factors of relation and position and not in- 

 trinsic factors located in any particular group of cells. In support of 



