THE MECHANISM OF LIFE 537 



while the unicellular organism needs to undergo either a scarcely 

 perceptible development or only a short cycle of changes, the 

 egg-cell must pass through an exceedingly long series of form- 

 changes up to the development of the multicellular organism. As 

 growth in the multicellular organism gradually ceases, the cells 

 undergo constantly fewer form -changes, and many tissue-cells, e.g., 

 the ganglion-cells, many of which do not grow at all in the adult 

 organism, remain apparently wholly unchanged, neither dividing 

 nor differentiating further. In reality, however, as has been seen 

 elsewhere, 1 general development never ceases wholly until the time 

 of death, but the later changes occur so very slowly and are 

 relatively so slight, that they are perceived only within long 

 intervals of time. In this apparently stationary condition the 

 tissue-cells are similar to those unicellular organisms that 

 have no perceptible development : in both, the correlation between 

 the internal and the external factors changes imperceptibly, in the 

 tissue-cells proceeding slowly, and in the unicellular organisms 

 being slight and constantly returning to its starting-point. 

 In neither are essential changes of form observed. 



From these considerations it appears how incorrect it is, from 

 the fact that the small egg is differentiated into a cell-structure 

 of astonishing complexity, to deduce the idea that the living 

 substance of the former in comparison with that of every other 

 cell, either every unicellular organism or every tissue-cell, must 

 be distinguished by an inconceivably delicate and complex 

 structure. This idea, which is met with very frequently, is 

 only an unrecognised relic of the doctrine of preformation 

 and, as has been seen, is both unnecessary and unjustified ; for 

 the development and differentiation of the cell-community 

 from the egg are based solely upon the correlation between the 

 living substance of the cells and the external factors, which is 

 continually changing with the continual growth and division of 

 the cells. Growth is the cause of all development, both of the 

 individual cell and of the whole cell-community, and this funda- 

 mental fact can scarcely be expressed better than in the words of 

 the old master of embryology, Karl Ernst von Baer ('28), who 

 thus stated the most general result of his studies upon the em- 

 bryology of animals : " The developmental history of the individual 

 is the history of the growing individuality in all its relations" 



A summary of the above considerations regarding the mechanics 

 of development leads to the following view. The developing cell, 

 like every cell, represents a drop of living substance, which is 

 characterised by a very definite metabolism. This metabolism is 

 the expression of the correlation existing between the medium 

 with its individual factors upon the one side, and the cell with its 

 manifold internal differentiations upon the other. By the growth 



1 Cf. p. 339. 



