374 l)(>V(.'Ki)iiiK'nt of the Lvnipliatie Xodos in the Pig 



Tho soeoiul point in advance is the formation of the germinal center. 

 AVithin the follicle, as will be seen in the diagram, there are small clnmps 

 of colls, definitely lymphocytes, heaped aronnd a capillary tuft. In the 

 entire node at this stage there are eight of these germinal centers. The 

 lymphocytes are closely packed in them, and there are more lymphocytes 

 near these centers than elsewhere in the node. In regard to the wan- 

 dering cells, the follicles contain in general four types : First, the lympho- 

 cytes which are found in the germinal centers almost to the exclusion of 

 any other free cell. A few of them are to he seen near the germ centers. 

 Second, polymorphonuclear forms, which are scattered throughout the 

 follicle except in the germ centers. Third, eosinophilic cells. Fourth, 

 mononuclear forms which have larger nuclei and more protoplasm than 

 the lymphocytes. All of these cells are found in the follicle. Eed blood 

 cells are also present, many of them being free in the connective tissue 

 meshes and showing a breaking up of the protoplasm into granules. This 

 appearance of the red blood cell is seldom seen in the corpuscles within 

 the capillaries. The bridges of connective tissue between the lymph 

 ducts have fewer wandering cells than the follicle. 



Beside the two changes already noted, namely, the division of the 

 artery and consequent multiplication of follicles, and secondly, the differ- 

 entiation of the follicle into germ center and lymph cord, there is a third 

 important change, namely, the beginning of the formation of lymphat- 

 ic sinuses out of the lymphatic ducts. It will be noted in the dia- 

 gram that around the border of the follicle the lymph ducts are arranged 

 in rows closely packed together and that the connective tissue bridges 

 between them are slender. This is still more definite in the next diagrams 

 given in Figs. 13 and l-l. In section, this point is to be seen in Fig. 15, 

 where the surface of the node adjacent to the capsule is made of a plexus 

 of lymph ducts, while in the depth of the node the ducts are closely packed 

 together in certain areas making sinuses. The section shows all grada- 

 tions in the width of the connective tissue bridges, some being just wide 

 enough to contain a single nucleus while others are broad bands. 



Kling was, I think, the first to note this method of the formation of 

 the sinus, though he clouded the clearness of his picture by saying that 

 subsequently the bridging of the sinus is made by the stretching of endo- 

 thelial cells across the lumen of the ducts, so that there are bridges made 

 of endothelium alone. This appearance is undoubtedly found in sections 

 just as in sections of the lung the epithelial lining of an air sac sometimes 

 shows as a membrane. 



By following the evolution of the sinus it is possible to understand 

 clearly the relation of the reticulum of the adult sinus to the endotlielial 



