HISTOLOGY OF THE THYMUS 317 



a vital staining of the granules with trypan-blaii. He found 

 that macrophages from lymph nodes, when filled with erythro- 

 cytes, no longer contained stainable granulae. In the cells under 

 consideration, the phagocytosis of other cellular elements is not 

 accompanied by a disappearance of the granules. 



There occurs then, in plasma cultures of frog thymus, a growth 

 and to a limited extent, a multiplication of large cells of varying 

 morphology, but evidently identical origin. The growth may 

 be in the form of fairly compact, tissue-like sheets of cells, in a 

 loose anastomosing reticulum, in long chains of cells joined end 

 to end, or the cells may be entirely isolated, rounded or with 

 plasmatic prolongations of varying tenuity and length. All 

 types of growth and transitions between them may be seen in 

 one and the same culture. The character of the nucleus in all 

 the cells is essentially the same, but its contour is naturally mod- 

 ified with the varying contour of the cell body. The cells all 

 contain granulae ranged upon a fibrillar ground- work, and demon- 

 strable both in vitally stained and in fixed preparations by a 

 variety of methods. The cells show from the first a tendency 

 to accumulate fat, and after this has reached a certain grade, 

 usually by the fourth or fifth day, further growth is retarded or 

 checked. The cells may be phagocytic towards the small thy- 

 mus cells. 



The probable nature and origin of these cells were not easy 

 to establish. Three possibilities suggested themselves: (1) that 

 they were derived from connective tissue cells, either from the 

 capsule or from the septa accompanying the blood vessels; (2) 

 that they were derived from the endothelial cells of the capil- 

 laries; or (3) that they were outgrowths of the epithelial reticulum 

 of the gland. The last view was the one finally adopted and 

 for the following reasons. Ihe growth could never be traced to 

 the capsule of the gland when portions were incorporated in the 

 tissue fragment. The connective tissue of the capsule showed 

 but slight capacity for growth, only a few fibrillated spindle cells 

 occasionally penetrating the clot for a short distance. Some- 

 times portions of striated muscle and connective tissue were 

 included in the fragment, but there was never any outgrowth of 



