HISTOLOGY OF THE THYMUS 309 



The degenerative changes which occur in the small thymus 

 cells are of two types. There may take place a simple lysis, in 

 which the cells become paler, losing their refractivity and finally 

 appearing as mere shadows. In fixed preparations, the nucleus 

 no longer stains and only a faint outline remains. More com- 

 monly the nucleus becomes smaller and more refractile and 

 stains intensely and diffusely with the nuclear dyes. Occasion- 

 ally, smaller particles are constricted off from the nucleus and 

 extruded from the cells, being found free in the plasma. Whether 

 the reduction in the size of the nucleus is accomplished wholly 

 in this way, is not certain. In fixed preparations of older cul- 

 tures, the majority of the small cells take on this form. The 

 bizarre radial extrusions, which are seen in the mammalian thy- 

 mus under circumstances leading to acute involutional changes, 

 were not often found in the cultures. This is rather surprising, 

 since the small cells of the thymus of infected or starving frogs 

 show these changes in marked degree. 



Under certain circumstances which were not accurately deter- 

 mined, there takes place an accumulation of fat in the small 

 cells. The most frequent appearance is the presence of three or 

 four rather large fat droplets in the cap of protoplasm corre- 

 sponding to the dell in the nucleus; but scattered droplets may 

 be found anywhere in the rim of protoplasm surrounding the 

 nucleus. In some preparations, almost every cell contained a 

 single larger droplet. 



No microchemical study as to the nature of this lipoid mate- 

 rial was made. The droplets are highly refractile and stain 

 brilhantly with Scharlach R. According to the studies of Holm- 

 strom (14) and Hart (15) on the rabbit and human thymus, fat 

 droplets, or lipoid granules demonstrable by Ciaccio's method are 

 normally absent from the small thymic cells; and Ciaccio (16) 

 has found that the lymphoid cells of the blood contain no recog- 

 nizable fat or lipoid substance. Stheeman (17) makes a similar 

 statement in regard to the lymphoid cells of the lymph-nodes. 

 From the above observation, however, one is forced to conclude 

 that the small cells of the amphibian thymus may, under certain 

 conditions, accumulate fat in visible form. Since the fat drops 



