310 GEO. A. THIEL AND HAL DOWNEY 



Further study of the same folUcle shows that many of the 

 small lymphocytes are formed in situ from reticular cells similar 

 in structure to the cells shown at c and d. A reticular cell iden- 

 tical and continuous with a, excepting that its nucleus has a few 

 larger chromatin granules, is shown at h. The cell c has a nu- 

 cleus similar to that of h with a sUght addition of chromatin, but 

 its cytoplasm is more basophilic and its processes have become 

 Yery delicate. At d both nucleus and cytoplasm are darker and 

 the cell is almost completely isolated from the general mesenchy- 

 matous network. The free cells e, /, g show further advance in 

 differentiation which is completed in h, a typical small lympho- 

 cyte. 



The cell k shows clearly that large lymphocytes may also be 

 cut off from the mesenchyme. The cytoplasm of this cell is still 

 continuous with the mesenchyme, but the structure of its nucleus 

 approaches that of the large lymphocytes I and m. As indicated 

 in the figure, large lymphocytes are extremely rare in the terri- 

 tory of the early follicles. They are far more numerous in the 

 pulp. The great majority of the free cells of the early follicles 

 are small and medium-sized lymphocytes. Among the former 

 are many cells whose nuclei have not developed the full amount 

 of chromatin characteristic of the small lymphocyte. With 

 Danchakoff, these cells might be classified as 'dwarf hemocyto- 

 blasts,' although it is evident that in the pig they are not products 

 of the proliferative activity of large lymphocytes (hemocyto- 

 blasts) similar to those in the pulp at I and m. Their nuclei are 

 clear, because the cells have only recently been cut off from the 

 reticulum and have not yet completed their differentiation. 



Figure 8 is a section of an early artery in the spleen of a 6-cm. 

 pig. The arterial wall is just beginning its differentiation — a 

 few elongated nuclei marking the initial stages of muscular devel- 

 opment in the media of the vessel. The vascular endothelium is 

 very distinct, its nuclei bulging into the lumen so that it is nearly 

 obliterated. In the mesenchyme surrounding the artery no dis- 

 cernible differentiation has taken place, but in other regions 

 throughout the mesenchymal tissue of the organ isolated groups of 

 hemocytoblasts are being differentiated into erythroblasts. Fig- 



