442 J. A. BADERTSCHER 



His views of the developing mammalian thymus are, therefore, 

 similar to Hammer's views of the developing Teleostean thymus. 



The chief exponents of the transformation theory are Prenant 

 ('94), Maurer in his later work ('99), Bell ('06)', Stohr ('06), 

 and Dustin ('11). The main point of this theory which they 

 most strenuously defend is that the lymphocytes arise from trans- 

 formed epithelial cells of the thymus. The epithelial cells 

 proliferate rapidly. A part of the daughter cells transform into 

 ' lymphocytes while the undifferentiated portions continue to 

 proliferate and are the source of succeeding generations of epithe- 

 lial cells and lymphocytes. All hold that the epithelial cells 

 give rise to the reticulum and Hassall's corpuscles, excepting 

 Dustin ('11) who claims that the reticulum is of mesenchjTnal 

 origin. These investigators regard the small round cells of the 

 thymus as real lymphocytes. 



Stohr also derived the small round cells of the thj^mus from 

 epithelial cells, but claimed that except for their sunilarity 

 in structure to the small lymphocytes in the blood, they have 

 nothing in common with them. They never enter the blood 

 stream, they remain epithelial cells as long as they exist, and 

 have the power to enlarge and change back to typical epithelial 

 .cells. In the medulla, according to Stohr, are found real 

 lymphocytes that have entered from the blood. He, however, 

 fails to explain how they can be distinguished from the small 

 round cells (epithelial cells) when they lie side by side. He also 

 derives the reticuhun and Hassall's corpuscles from the epithelial 

 cells of the thymus. 



It is now generally accepted that the thymic bodies are derived 

 from the epithelial cells of the thymus. Hammar and Bell 

 have given a thorough description of their development. 



From this brief historical sketch it can be seen that the nature 

 of the developnient of the thymus is by no means a settled ques- 

 tion. I wish to state at this point that the results of my investi- 

 gation of the histogenesis of the thymus agree with these of Ham- 

 mar ('08) and Maxim ow. 



A brief historical sketch relative to the origin and develop- 

 ment of the free erythrocytes and granular cells of the thymus 

 will be given in connection with their consideration. 



