THEORIES AND PROBLEMS OF CANCER 677 



in the ordinary course of events, after differentiation has taken 

 place, cells tend to produce like cells and do not produce any- 

 other type. In the higher animals, after differentiation has 

 taken place, the cells are incapable of producing any cells but 

 those similar to themselves. The formation of scar tissue after 

 an injury is an exception in a sense but one that does not affect 

 the general rule. As we go lower in the animal kingdom, we 

 find that the cells that have become highly differentiated possess 

 more and more of the power to produce cells which are unlike 

 themselves. A remarkable instance of this is afforded by some 

 experiments upon the eyes of salamanders and newts. The 

 earlier generations of cells produced from the fertilised ovum 

 are arranged in two layers, the epiblastic and the hypoblastic ; 

 later on a third group is produced between these two layers, 

 the mesoblastic cells. Now the crystalline lens of the eye is 

 produced from an ingrowth of epiblastic cells which are very 

 highly differentiated, being flattened and transparent and quite 

 unlike any cells produced from the mesoblast. But if the lens 

 be removed from the eye of the salamander or newt, a new lens 

 is produced from mesoblastic and not from epiblastic cells. 1 In 

 mammals, however, such a power does not exist, some groups 

 of cells, e.g. those forming the skin, being capable of multiplying 

 and producing other differentiated cells like themselves but 

 never any cell unlike themselves. As we approach the lowest 

 multicellular animals, we find this general potentiality retained 

 in greater force and the whole body may be reproduced from a 

 half or even smaller part, as in Planaria or Hydra. In plants, 

 even in the later stages of evolution, this potentiality is often 

 retained in a high degree, so that a whole individual may be 

 produced by a simple process of growth from a small group 

 of already fully differentiated cells which have been forcibly 

 separated from the parent organism. It would appear that, 

 though somatic co-ordination still exists, in all these cases where 

 a high degree of general potentiality is retained by the differen- 

 tiated cells, its sway has not become so absolute as it is in 

 the case of vertebrates. There is in fact a regular gradation 

 from cases in which a small specialised portion of an organism 

 is capable, when cut off, of continuing a separate existence and 

 developing all the parts of the parent organism, to cases in 



1 Wolff, Gustav, Archiv f. Eniwick. i. 3, 1895; Miiller, E., Archiv /. mikr. 

 Anat. xlvii. i, 1896. 



