440 J. A. BADERTSCHER 



fluid (used by Bell) the basophilic character of the cytoplasm 

 of the lymphocytes is lost, thus rendering it diflricult to distin- 

 guish a large or medium sized lymphocyte from some of the 

 smaller epithelial nuclei of the thymus. Mallory's connective tis- 

 sue stain was also used for staining the connective tissue fibers of 

 the thymus. 



HISTORICAL 



Only a brief historical sketch will be given to outline, in a 

 general way, the views regarding the histogenesis of the thjmius. 

 For a comprehensive review of the literature on this subject 

 reference should be made to Hammer's work of 1910. 



The investigations that have been made of the histogenesis 

 of the thymus of various classes and species of animals have 

 led to the formation of two general theories, viz., the pseudo- 

 morphosis theory and the transformation theory, each of which 

 has been more or less modified by the difi"erent investigators of 

 this subject. As it is beyond the scope of this work to give a 

 detailed discussion of each theor}^ and its modifications they 

 will be discussed only in a general way. The pseudomorphosis 

 theory will be first considered. In its original setting this theory 

 held that the epithelial anlage of the thymus is gradually in- 

 vaded by mesenchymal and adenoid tissue. This process dis- 

 places the epithelial cells and the only remnants of them in the 

 fully developed thymus are the Hassall's corpuscles. This view 

 was held by Maurer for the thymus of teleosts ('86) and for the 

 thymus of Urodela and Anura ('88). He gives no detailed 

 description of the development of the reticulum, or of the 

 invasion of the epithelial anlage of the thymus by the lympho- 

 cytes. 



Von Ebner held a somewhat modified view of the pseudo- 

 morphosis theory as set forth above. According to him, the 

 reticulum and Hassall's corpuscles of the medulla are derived 

 directly from the cells of the original epithelial anlage, while 

 the entire cortex with its reticulum, lymphocytes, and blood 

 vessels, and also the lymphocytes of the medulla, are of mesen- 

 chymal origin. 



