THE MOSAIC STAGE OF DIFFERENTIATION 



215 



ence of what has here been called the mosaic stage of differentiation 

 and development. This is of great theoretical interest, since it shows 

 that the capacity for regulation, which has been regarded by some 

 authors ^ as a universal property of life, does not hold at all for an 

 important stage of development, universally passed through by all 

 higher animals. 



'0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 



Days 



Fig. 103 

 Physiological changes accompanying metaplasia. Solid line throughout (401-3), 

 growth-curve of a flask culture of fibroblasts from embryo chick muscle ; although 

 grown in an unfavourable medium (lacking embryo extract), the culture showed 

 no metaplasia. 104-1 , growth-curve of sister-culture under similar conditions, in 

 which metaplasia fibroblasts to macrophages occurred on the eighth day. The 

 subsequent growth (dotted line) was much more rapid. (From R. C. Parker, 

 Journ. Exp. Med. lviii, 1932.) 



§4 

 Turning now to the question of the time of onset of the mosaic 

 stage of development, we must refer to the classical experiments on 

 so-called mosaic-eggs, referred to in Chap. v. They serve as a 

 further illustration of the principle which is here under discussion ; 

 in their case, the onset of chemo-differentiation has merely been 

 transferred to an earlier stage of development. 



^ Driesch, 1921 ; J. S. Haldane, 1929. 



