THE MOSAIC STAGE OF DIFFERENTIATION 211 



considering : but the reactions of the fibroblasts to this process are 

 purely passive ; and if removed from the local influence, they simply 

 retain the characteristics impressed up to the time of isolation. This 

 we may regard as a new type of dependent differentiation : mesen- 

 chyme is predetermined to differentiate into fibroblast tissue, but 

 the detailed characteristics of the fibroblasts are impressed from 

 without. 



It is of interest to note that many of the characteristics of tissues 

 are dependent on specific physiological characters of the cells them- 

 selves. In tissue-cultures, fibroblasts form an irregular matted 

 tissue; epithelial cells associate with one another in an orderly 

 manner; amoebocytes remain separate and never form a compact 

 tissue. These determined types of cell behaviour persist indefinitely 

 in vitro. 



Cells which normally form part of a more highly differentiated 

 tissue possess and retain the type of behaviour which leads to the 

 formation of such tissue. Thus, kidney cells can redifferentiate into 

 kidney tubules,^ and capillary cells can redifferentiate into capillaries 

 in vitro ^^ 



Under certain circumstances, however, it appears to be possible 

 for cells to undergo a permanent and irreversible change in type 

 and characters, comparable in its way to the changes observed in 

 somatic mutations in vivo. This phenomenon, known as metaplasia^ 

 has from time to time been claimed to occur in many cases of re- 

 generation, when it has been asserted that certain structures have 

 been formed from cells of a different tissue. It is, however, often 

 difficult if not impossible to be sure that undifferentiated and em- 

 bryonic cells were not present, and that the differentiation of the 

 structure in question did not proceed from them. This possibihty 

 seems to be excluded in the regeneration experiments performed 

 on Nemertines. In these animals, there is a certain region at the 

 anterior end of the body in front of the mouth, which contains no 

 endodermal tissue at all. If such a piece be isolated, it will recon- 

 stitute itself into a complete worm, with an alimentary canal which 

 quite certainly, therefore, is derived from cells of an entirely 

 different tissue.^ 



^ A.H.Drew, 1923. ^ Lewis, 1931. 



^ Nussbaum and Oxner, 19 10. 



14-2 



