THE MOSAIC STAGE OF DIFFERENTIATION 213 



Metaplasia has been observed to occur as a result of certain graft- 

 ing experiments. If a small piece of the ripe ovary of the Mollusc 

 Pecten is grafted into the adductor muscle of another individual, 

 the implant rapidly becomes surrounded by a layer of fibroblasts. 

 The grafted tissue degenerates and is destroyed by phagocytosis, 

 but the fibroblasts remain, forming the lining of a cyst containing 

 the debris. After three weeks, the fibroblasts begin to take on the 

 appearance of columnar epithelium, which eventually becomes 

 ciHated.i It is almost impossible to believe that undifferentiated 

 ciliated cells were originally present in the muscle, and we are ac- 

 cordingly forced to regard this case as one of true metaplasia (fig. i oi ) . 



Tissue-culture experiments likewise provide evidence for meta- 

 plasia. Monocytes which have been treated with filtered extracts of 

 a particular type of tumour (the Rous sarcoma) become transformed 

 into fibroblasts.^ The crowding of the cells in the culture often 

 produces the same effect, whereas various modifications of the 

 medium fail to do so. The change into fibroblasts is of an adaptive 

 nature, occurring when conditions are becoming impossible for the 

 continued existence of monocytes.^ This transformation may be 

 permanent.^ On the other hand, fibroblasts treated with plasma 

 containing liver extract may become transformed into macrophages 

 with all their physiological characteristics, which they now keep 

 indefinitely.^ 



Cultivation of fibroblasts in a plasma medium which only permits 

 of their slow growth may also induce metaplasia into macrophages. 

 Here again, the change to the outwandering macrophage type is 

 probably adaptive. Even Carrel's 20-year old strain of fibroblasts 

 has been made to produce daughter-strains of macrophages in this 

 way (fig. 102). 



The rate of growth of the macrophages is markedly superior to 

 that of their parent fibroblasts ; they appear to retain their charac- 

 teristics indefinitely^ (fig. 103). 



Lastly, it may be mentioned that the obscure changes which 

 tissues undergo when tumours and_^cancers arise are of the nature 

 of metaplasia. The morphological characters of the cells are lost to 



^ G, H. Drew, 191 1. 2 Carrel, 193 1. 



^ Carrel and Ebeling, 1926. * Fischer, 1925. 



^ R. C. Parker, 1932 c. 



