304 ALWIN M. PAPPENHEIMER 



distributed chromatin network, which is not in the form of large 

 discrete clumps. The cell limits cannot ]je made out clearly, 

 even in very thin sections (2^)- Occasionally in the cortical 

 portion, the small thymic cells show a distinct outline, the nu- 

 cleus being bounded by a narrow rim of brownish protoplasm, 

 which is thicker at one pole of the cell, where the nucleus com- 

 monly shows a slight dell or indentation. In the medulla, where 

 the different types of cells are intimately commingled, the 

 nuclei appear separated by an indefinite protoplasmic substance. 

 It is thus exceedingly difficult to determine to which cell the 

 cytoplasmic granulae belong. Scattered between the nuclei, 

 but never in or upon them, are very numerous minute purple 

 granulae (fig. 1). These vary somewhat in size, but are in 

 general smaller than the smallest coccus, and in some cells are 

 barely within the range of visibility. (Comp. OC/6, Imm. 

 1/12 in.) The abundance of the granulae depends largely upon 

 the degree of differentiation in acetic acid and alcohol. In 

 sections which have been well differentiated, the granulae are 

 fewer in number, but are more distinct, standing out sharply 

 from the pale brownish background. 



Instructive pictures are found near the ragged edge of sections 

 in areas in which the small thymic cells have been dislodged or 

 fallen out, and only the protoplasmic reticulum persists. Here 

 the protoplasmic meshwork is found studded with innumerable, 

 minute sharply defined granulae. 



Whether the small thymic cells also contain granulae, or 

 whether these are limited to the reticular cells, is difficult to 

 decide from a study of the sections alone. The granulae are 

 present in the cortical portion of the gland, in some sections in 

 considerable abundance. Often they are in close relation to the 

 nuclei of the small thymic cells, and in favorable cases, where 

 the cells have become separated by a break in the section, they 

 appear to lie within the narrow zone of protoplasm on one side 

 of the nucleus. Often they form a row lying between adjacent 

 cells, and they adhere to, or are incorporated with individual 

 cells which have become loosened from their surroundings. From 

 a study of the sections, the impression was gained that the small 

 cells as well as the large reticular epithelial elements, contain 



