298 GEO. A. THIEL AND HAL DOWNEY 



On the inner surface is an endothelium, the cells of which possess 

 but scanty amounts of protoplasm and large nuclei that extend 

 into the lumen of the capsule. In some cases this condition is so 

 pronounced that the lumen is almost completely obliterated. 

 The capsule itself consists of compact tissue with many irregular 

 nuclei and no distinct cell walls. Many very fine fibers, so ar- 

 ranged that they are often mistaken for cell walls, lie parallel 

 with the vessel, but no elastic fibers are present. The capsule is 

 cut off from the reticulmn by a rather distinct band of fibrous 

 tissue, some of the fibers of which extend into the reticulum. 

 In the outer zone are nmnerous leucocytes, and throughout 

 the entire capsule spaces unlined by endothelium are filled with 

 deformed erythrocytes and a few scattered leucocytes. 



Weidenreich is not positive concerning the nature of the cells 

 which make up the capsule. He finds that the structure of the 

 capsules is sunilar to that of the inner fibrous membrane of the 

 larger arteries described by Henle, and believed by him to be 

 derived from the endothelium. The capsule is not a thickening 

 of the adventitia nor a continuation of the media, for the muscle 

 cells of the latter are of very different character. Weidenreich 

 cannot agree with Carlier that the capsules are made up of reticu- 

 lum, for the tissue does not resemble reticulum in appearance and 

 the nuclei are much more irregular than those of the reticulum. 

 He is inclined to agree with Kolliker's view that the endothelial 

 and capsular cells are derived from a common cell-form. 



Sabin agrees with Bannwarth in maintaining that the ellip- 

 soids or capsules appear before the follicles and disappear in later 

 embryonic life when the follicles make their appearance. ' ' Thus 

 the ellipsoid is the prmiitive lymphoid structure of the spleen." 



In the spleen of pig, the development of the capsules is initi- 

 ated in 15 to 17-cm. embryos. They are recognized as slightly 

 condensed areas of the mesenchyme, penetrated by distinct 

 arterial capillaries or arterioles. The only relation they bear to 

 the early splenic follicles lies in the continuity of a common 

 mesenchymal reticulum from which both are differentiated, but 

 at sharply separated regions along the arterial wall. 



