MORPHOLOGY OF ANIMALS 



(a) in plants and animals now living on the earth, (b) in 

 the history of organisms through past geological eras, (c) 

 in the development of animals and plants from egg cells 

 to their adult condition. 



2. Methods of Differentiation. The method by which in- 

 creasing differentiation is brought about is best seen in the 

 process of individual development. Here the single egg 

 cell divides into many cells which come to differ from 

 one another in both structure and function, {a) In this 

 process there is a distribution of different structures to 

 different cells and a concomitant distribution of functions 

 among these cells; that is, there is "morphological di- 

 vision of substance" and "physiological division of labor." 

 {b) But increasing differentiation does not consist mere- 

 ly, nor chiefly, in the division of structures and functions 

 already present and their segregation into separate cells, 

 but also in the appearance of new structures and functions 

 in the later stages that were not present in the earlier ones. 

 These new structures and functions were not created ex 

 nihilo^ but have come from new combinations of factors 

 already present, that is from transformation of old struc- 

 tures and functions rather than from new formation. 

 These new differentiations of development are, therefore, 

 the results of "creative synthesis," just as water with its 

 many peculiar properties is the result of the chemical 

 synthesis of two atoms of hydrogen and one of oxygen. 



3. Structure and function are not separate or independent 

 entities, they are merely two aspects of one thing, namely, 

 organization. From the morphological point of view a 

 living thing is a system of structures, from the physio- 



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