484 J. A. BADEETSCHER 



protoplasmic processes have been withdrawn. The phagocytes 

 vary greatly in size. Some are from two to three times as large 

 as the largest lymphocytes. Only a few were found in the lobules 

 of the thymus of late developmental stages. 



Cysts were found in the thymus in embr3^os 55, 65, 110, 125, 

 165 and 180 mm. in length. They vary in size and shape 

 and all are lined with simple cuboidal or low columnar epithelium 

 which is ciliated only in patches. The cilia are long and slen- 

 der. No consideration was given to their origin. 



CONCLUSIONS 



1. The lymphocytes first present in the thymus are all large 

 lymphocytes and have migrated into it from the mesenchyme. 



2. The numerous small round cells of the thymus are formed 

 by the repeated division of the large lymphocytes which thus 

 become small, and also by their own proliferation. 



3. Judging from the source and structure of the small round 

 cells they are small lymphocytes and are identical with the small 

 Ijrmphocytes of the blood. The thymus, therefore, may well 

 be considered as a source of some of the small lymphocytes found 

 in the circulating blood. 



4. The reticulum of the thymus is of epithelial origin and is 

 formed passively by its meshes becoming filled with lympho- 

 cytes which separate the nodal nuclei farther apart and thus 

 greatly attenuate the protoplasmic processes of the syncytium. 



5. The Hassall's corpuscles are of epithelial origin. 



6. The free red blood-cells and eosinophile cells found in both 

 interlobular septa and the thymic lobules are derived from Ij^m- 

 phocytes in situ. 



7. Whether or not any of the erythrocytes formed in the 

 thymus enter the circulating blood is difficult to determine in 

 fixed material. Some of the free erythrocytes undoubtedly 

 undergo degeneration and the products of disintegration of those 

 existing in the form of eosinophile granules are taken up by the 

 lymphocytes which thus become transformed into eosinophile 

 leucocytes. 



