DEVP^LOPMENT OF THE THYMUS 475' 



erythrocytes, and in groups of nucleated red cells they are almost 

 invariably found scattered among the erythroblasts and normo- 

 blasts. Typical large erythroblasts and normoblasts such as 

 occur in the thymus of the 55 mm. stage are also present in the 

 thymus of late developmental stages; but in these stages they 

 make up only a small proportion of the erythroblasts. This iS' 

 accounted for by the fact that the small nucleated red cells are 

 derived from the small lymphocytes while the large ones are 

 derived from large and medium-sized lymphocytes which are 

 comparatively few in number in late developmental stages. 



The debris of degenerated nuclei extruded from the normo- 

 blasts can often be found scattered among erythrocytes. It 

 is not uncommon for this debris to collect in heaps which appear 

 in sections as deeply stained dark structureless patches in a group 

 of erythrocytes. Why the degenerated nuclei have a tendency 

 to flow together can only be conjectured; also why the red blood- 

 cells are mostly formed in groups instead of uniformly through- 

 out a lobule is unknown to me. 



The blood stream of a 55 mm. embryo contains nucleated 

 red blood-cells and since capillaries are already found in the 

 thymus in this developmental stage it might still be argued,' 

 as is held by some, that the free erythrocytes in the thymus are 

 derived from the blood. This, however, cannot be the case for 

 in a full term fetus in which many erythrocytes and nucleated 

 red blood-cells are found in the parenchyma of the thymus no 

 nucleated red cells were found in the blood stream. 



From the observations cited above I must conclude that the 

 erythrocytes in the meshes of the reticulum of the thymus are 

 derived from the lymphocytes. Furthermore, on this con- 

 clusion are hinged three important features two of which strongly 

 reflect on the nature of the small round cells of the thymus, 

 while one furnishes additional proof for the pseudomorphosis 

 theory of the histogenesis of the thymus. They are: (1) Since 

 both the lymphocytes of the mesenchyme and the small round 

 cells of the thymus have the power to transform into erythro- 

 cytes they are potentially alike. The small thymic cells are, 

 therefore, Ijonphocytes and not epithelial cells as held by Stohr 



