Karyokinesis 33i 



Here at the end of the series may be found one of the 

 secrets of evolution. These cells, which make the most 

 recent features, are susceptible of modification by environ- 

 ment more rapidly than the earlier cells of the middle of the 

 division process. The early cells must develop as their 

 ancestors did; they are committed to that purpose, by 

 heredity established so long that they cannot change, but 

 the latest cells, which specialized recently when the structure 

 was near completion, may vary much in development, and 

 may even show longer or shorter continuation of division 

 development, as conditions give impulse. Such latest cells 

 are not necessarily the external cells of the skin or hair. 

 Those of nerves and brain and of interior membranes in- 

 clude late developments of this class. Broadly speaking the 

 development is most set and unchangeable in its earlier 

 stages and most variable in its latest. 



