PROBLEMS AND MATERIAL 7 



to an orderly pattern; that is, a progressive organization occurs. The 

 problem of organization is the problem of the origin and nature of the 

 pattern underlying and determining where, when, and how the differences 

 appear and in what they consist. It involves the physiology and behavior 

 of living protoplasms. 



PHYSIOLOGICAL DIFFERENTIALS OR GRADIENTS 



Cells commonly have rather sharply defined boundaries, but in the 

 earlier stages of developmental pattern sharply defined boundaries be- 

 tween the various regions or fields of the developing organism are usually 

 absent. Instead of such boundaries, we commonly find evidences of grad- 

 ed differences from one region to another. These differentials, gradations, 

 or gradients appear in various aspects. For example, we find gradations 

 in visible protoplasmic structure and in physical and chemical constitu- 

 tion; in rate of cell division; in cell size; in rate of growth, morphogenesis, 

 and reconstitution ; in degree of determination of organs; in respiration; 

 and in reaction to various external factors. Where it has been possible to 

 compare a number of these different gradients in the same individual, a 

 high degree of correspondence as regards region involved and direction has 

 usually been found. The most conspicuous of these gradients and the first 

 to become evident in development are those associated with the polarity 

 and symmetry of the whole organism. In fact, a gradient of some sort is 

 very commonly, if not always, the earliest distinguishable evidence of a 

 physiological axis. Similar gradients also appear in the early developmen- 

 tal stages of various organ systems and organs. Moreover, localized de- 

 velopmental activities very generally show evidence of a graded decrease 

 from a region of highest intensity in a certain or in all directions. It has 

 been possible, in many cases, to alter or obliterate such gradients and to 

 determine new ones experimentally and so to alter the axiate pattern of 

 development correspondingly in definite, controllable manner. The ex- 

 perimental data which will be considered in later chapters show, beyond 

 question, that various sorts of gradients which appear in development are 

 manifestations or expressions of underlying physiological differentials of 

 some sort, which are organismic in order of magnitude; that is, they coin- 

 cide with the scale of organization of the individual or part concerned. 

 In short, these physiological gradients are characteristic features of axiate 

 order and pattern : differentiation of regions and organs occurs in definite 

 relation to them. Obviously, they are associated with development in 

 some way, cither as factors which arc operative and determinative or as 



