TISSUE CULTURE 79 



While the character of the medium is apparently the chief 

 factor causing structural variation in tissue-culture cells, others 

 play their part. Monocytes, like blood leucocytes of which they 

 are a variety, are scavenger cells. After filling themselves with 

 degenerate cells and other waste matter, they become many 

 times larger and are then known as macrophages. The macro- 

 phages are grown-up or, shall we say, filled-up monocytes. 

 They may take in as many as 50 cells of their own size. Such 

 transformations have been found by W. H. Lewis to be of very 

 common occurrence in cultures of blood from a variety of 

 animals. It is not, perhaps, quite correct to say that gorging 

 brings about the transition of monocytes into macrophages but 

 rather that "giant" cells arise from either of the two, which are 

 different forms of a very plastic basic type of cell. Monocytes 

 and macrophages may also arise from and become transformed 

 into fibroblasts. Carrel and Ebeling found that fibroblasts, 

 treated with plasma and heparin, may become macrophages. 

 They acquire all the physiological properties of macrophages and 

 remain indefinitely in that state. The macrophage and the 

 fibroblast are therefore apparently functional variations of the 

 same basic cell type. 



This seeming lack of stability in type of cells in tissue culture is 

 offset by a strong tendency of cells to remain true to type, at 

 times. Ebeling and Fischer have shown that strains of epi- 

 thelium and fibroblasts cultivated side by side in the same 

 medium keep definite individual characteristics even though 

 they may change in external form. Thus, certain cells which are 

 morphologically (structurally) indistinguishable from fibro- 

 blasts retain, after many months in culture, original physio- 

 logical properties not possessed by fibroblasts. Vogelaar has 

 grown embryonic human liver which, though in culture (Fig. 61), 

 shows a pronounced tendency to produce a network and form 

 typical trabeculae which are structural features characteristic 

 of the adult liver. Many of the cells possess two nuclei, 

 another property typical of liver (epithelial) cells. 



Heredity and environment determine the destiny of living 

 organisms. By heredity is meant that physical and chemical 

 constitution that the cell acquires when it is first formed. Envi- 

 ronment involves many factors, from temperature, moisture, and 

 salts to the presence of other cells and the position of a cell in 



