HEREDITY 



Stages of embryonic development, cells which would have 

 produced parts of the central nervous system will produce 

 skin if transplanted to a region that would normally 

 produce skin. An organization center exists at a given place 

 in the hollow, ball of xells comprising the gastrula, and 

 what each cell becomes depends upon its position with 

 reference to this center. The organization center itself can 

 be shifted experimentally, and, following the change, the 

 whole organization pattern of the embryo is altered. 



In later stages of development this formative power is 

 lost. Differentiation having passed a certain point, the 

 fate of any given structure is more firmly fixed. Liver cells, 

 or spleen cells, or brain cells, or skin cells remain the cells 

 of these types, no matter where they may be placed. Yet 

 an opportunity for a limited degree of variable develop- 

 ment still exists in individuals having identical gene 

 patterns. 



In the first place, the developmental machinery is not 

 perfect. Slips may occur; and from these slips some diversity 

 of structure and function may ensue. These workshop 

 errors may affect organs having little influence on general 

 physiological activity, and the result be negligible; but 

 if they retard or accelerate the energy of such essential 

 organs as the ductless glands, the consequences may be 

 more serious. In the second place, an alteration in the 

 physical environment may change the rates at which 

 chemical changes take place, and thus entail poor develop- 

 ment or superior development, as the case may be; but the 

 changes induced by such causes are quantitative rather 

 than qualitative. In fact, one should not expect to have 

 the normal development pattern upset markedly by either 

 of these causes. An organism is a dainty piece of machinery; 

 and the higher it is in the evolutionary scheme, the more 

 delicate it turns out to be. If slips in development occur, 

 therefore, or if inhospitable conditions are met, growth is 



[177] 



