Facts and Factors of Development 31 



as development advances each of these functions becomes more 

 specialized, more complicated and more perfect. A cell which at 

 an early stage was protective, locomotor and sensory in function 

 may give rise to daughter cells in which these functions are dis- 

 tributed to different cells ; cells which at an early stage were 

 sensitive to many kinds of stimuli give rise to daughter cells which 

 are especially sensitive to one particular kind of stimulus, such as 

 vibration, light, or chemicals. 



Differentiation of Functions and Structures. Functions de- 

 velop from a generalized to a specialized condition by the process 

 of "physiological division of labor" which accompanies morpho- 

 logical division of substance. But just as in the union of hydro- 

 gen and oxygen a new substance, water, appears which was not 

 present before, by a process of "creative synthesis," and as in 

 the development of structures new parts appear, which were not 

 present in the germ, so new functions appear in the course of de- 

 velopment, which are not merely sorted out of the general func- 

 tions present at the beginning, but which are created by the in- 

 teraction and synthesis of parts and functions previously present. 

 For example, Lane has shown that young rats are quite insensitive 

 to light until several days after birth although the eye begins to 

 form at a very early stage of development. Doubtless every part 

 of the eye is functioning in one way or another during the entire 

 development but not until all parts are formed and connected 

 and all their functions are synthesized does the new function, 

 vision, spring into existence. Undoubtedly the same is true of 

 many other complex functions which have no existence until all 

 their constituents are present and integrated, when they sud- 

 denly appear. 



Living Functions and Structures Inseparable. Much less at- 

 tention has been paid to the development of functions than to the 

 development of structures, and consequently it is not possible to 

 describe the former with the same degree of detail as the latter. 

 But in spite of the lack of detailed knowledge regarding the de- 

 velopment of particular functions the general fact of such devel- 



