172 JOHN STEPHENS LATTA 



assume no definite position in the nodules, but are found scattered 

 throughout (figs. 3 and 4). 



As the number of lymphocytes in a nodular mass increases, the 

 connective tissue surrounding it tends to become pushed back and 

 piled up or condensed apparently by outward pressure of the 

 increasing number of lymphocytes, and the fibers and connective- 

 tissue cells (fibroblasts) tend to assume a, circular or concentric 

 arrangement about the mass of lymphocytes, thus giving rise to 

 a fairly dense sheath or capsule, which gives the nodule more defi- 

 nite limits and outline. 



This sheath or capsule is by no means as complete and defi- 

 nite a structure as is found, for example, in the capsule of lym- 

 phatic nodes, but is rather a physical expression of the proliferation 

 of lymphocytes in regions of unusually rich vascularity. Lympho- 

 cytes are always found to some extent among and outside of the 

 connective-tissue fibers of the sheath, being by no means strictly 

 confined within it. 



In the process of the proliferation of lymphocytes and the for- 

 mation of the connective-tissue sheath, reticular fibers and cells 

 are gradually forced outward from the center of the mass, so that 

 in later stages of nodular development few or no fibers or reticu- 

 lar cells can be demonstrated except near the periphery of the 

 nodule. 



Although the nodular sheath does not form an impassable 

 barrier for the passage of lymphocytes, its presence and the fact 

 that the reticulum is so much closer meshed at the periphery tend 

 to inhibit the spreading of lymphocytes which are rapidly increas- 

 ing in number. This inhibition causes the lymphocytes to become 

 more closely packed at the periphery of the nodule than in the 

 center. Thus the center of the nodule, because of the lesser 

 number of cells there, appears lighter than at the periphery. This 

 lighter appearance of the center of the nodule was noted by 

 Flemming ('85) and named by him the ' Keimzentrum, ' or 

 'germinal center,' for he considered this lighter central area a 

 center of proliferation of lymphocytes. Later investigators have 

 followed Flemming in calling this area the germinal center of the 



