320 ALWIN M. PAPPENHEIMER 



thelial tumors, might be expected. It is nevertheless interesting 

 that the tissue growing under highly artificial conditions, should 

 conform so closely to its normal type, and in the phagocytosis 

 of the small cells, exercise a function which is characteristic of 

 the normal thymic reticulum. 



Before discussing the conclusions to be drawn from the fore- 

 going observations, I wish to record briefly certain further studies 

 on the comparative growth of thymus and lymph-nodes. The 

 tissues for these experiments were obtained from young adult 

 rats and incubated at 37°. 



The small thymic cells emigrate rapidly, often reaching the 

 edge of the plasma drop within a few hours. The lymphocytes 

 of the lymph glands behave in the same way. Degenerative 

 changes begin rapidly, both in the thymic cells and in the lympho- 

 cytes. The nuclei become pycnotic, fragment, and finally break 

 up into globular, deeply staining particles. Well preserved, 

 actively amoeboid cells were not found after forty-eight hours. 



Growth from the thymic fragments begins usually on the 

 second day, as fusiform or polygonal cells with intercellular con- 

 nections. Large flat cells on the cover-glass resemble those 

 described in the frog's thymus ; but there is frequently a radially 

 directed growth of long spindle cells resembling connective tissue. 

 After three or four. days, however, there is usually a tendency 

 towards the formation of flat cellular planes, the growing margin 

 of which is very definite and sharply limited. A few cells at 

 the periphery may become partially or completely separated, 

 but the growth on the whole is coherent (fig. 8), and often ap- 

 proaches in its character the growth of epithelial tissue from 

 carcinomata, as described by Lambert and Hanes (36) and L. 

 Loeb (38). 



At this stage of the growth, moreover, the thymus culture 

 shows changes which distinguish it definitely from the culture 

 of lymph-node. The central fragment becomes rarefied, and 

 there appear large numbers of globular cells of large size, which 

 with the low power seem filled with coarse granules. These 

 granules are really ingested small cells in various stages of pyc- 

 nosis and degeneration. Scattered phagocytic cells of this type 



