THE MECHANISTIC CONCEPTION 37 



a fully developed cell never changes its character 

 under normal conditions. Now it is also true that 

 the different adult cells have different origins in an 

 embryological sense. Early in development there 

 is a well-known differentiation of the segmented egg 

 into three distinct layers or skins. From the inner 

 skin come the epithelial cells of the digestive tract 

 and digestive glands. From the middle skin come 

 the bones, cartilages, connective tissues, muscles, 

 and blood. From the outer layer come the epithelial 

 cells of the skin and, by a kind of embryological 

 jugglery, the nerve cells of the central nervous 

 system. This predestination of cell character is in 

 reality foreshadowed at a still earlier period by ter- 

 ritorial differentiations which make their appearance 

 soon after the impregnation and cell division. We 

 have, indeed, no means of knowing how early in the 

 history of the germ plasma the future of the different 

 cell types is definitely fixed. One thing, however, 

 is quite clear. The governing cells of the body 

 the nerve cells have their origin in materials 

 destined to become nerve cells. Like other varieties 

 of cells, they form a kind of caste. The democratic 

 or essentially interchangeable feature which is 

 found in republican forms of government is lacking 

 here. In the animal organism those materials best 

 fitted by nature for government find their way to 

 the controlling power, while those materials best 

 fitted to serve in other ways move with equal 

 precision towards their destiny. It is thus clear 



