HISTOLOGY OF THE THYMUS 313 



Often a fibrillar structure of the cytoplasm is recognizable in 

 the living cell, the fibrils being especially distinct in the long 

 slender protoplasmic processes. The appearance of these cells 

 and their usual manner of growth, is indicated in figures 5, 6 

 and 7. 



Changes of contour and relative position are readily detected 

 in these cells, if the observation be sufficiently prolonged. They 

 do not, however, under normal conditions, show as rapid changes 

 of shape, as do the cells of the lymphoid type. After the first 

 days of active emigration and growth, they retain their position 

 and outline unchanged for many days, gradually accumulating 

 fat drops in their protoplasm. 



An interesting phenomenon which was seen in the living cells, 

 and confirmed by subsequent fixation and staining, is the sepa- 

 ration of portions of protoplasm, which gradually become con- 

 stricted off and are set free into the plasma. The separated 

 portions contain fat drops and granulae. The process resembles 

 curiously the formation of blood platelets from megakaryocytes, 

 as first described by H. Wright (25). Its significance here is 

 uncertain; it may be found in cells which show no other degen- 

 erative changes. 



The stained preparations of these cultures give somewhat vary- 

 ing pictures according to the method of fixation. After formalin, 

 followed by iron-hematoxylin (Heidenhain), the nuclear sub- 

 stance stains homogeneously, the intensity depending upon the 

 extent of decolorization. There are one or two nucleoli of large 

 size, deeply stained, but often with a slightly paler center. There 

 are in some of the nuclei, minute granules surrounded by a clear 

 halo. The cytoplasm shows a beautifully reticulated structure, 

 being composed apparently of delicate fibrillae, occasionally 

 parallel in their course, but disarranged by the presence of smaller 

 and larger fat vacuoles. Along the meshes of the reticulum are 

 deeply staining granulae of smaller and larger size. Most of 

 them are larger than the granules of the eosinophile cells. They 

 are usually spherical, but may be slightly elongated, or when in 

 apposition to a large fat globule, crescentic. The numbers vary; 



