GRADIENTS, FIELDS, AND DETERMINATION 297 



structure appears unnecessary and, if present, is a result, not a cause, of 

 physiological polarity. Third, the question how such a molecular structure 

 can bring about organization and morphological differentiation remains. 

 It is suggested by Harrison (1937) that the polar, symmetrical, or asym- 

 metrical structure of protein molecules brings about the localization of 

 different substances at opposite poles of the egg or in relation to sym- 

 metry or asymmetry. Developmental pattern, according to these views, 

 is primarily structural and static. Recent X-ray diffraction photographs 

 of various embryonic tissues of amphibians and the chick have failed to 

 show any evidence of molecular orientation that might constitute a basis 

 for spatial developmental pattern. The authors, however, point out that 

 these negative results do not prove the absence of such orientation (Har- 

 rison, Astbury, and Rudall, 1940, "An attempt at X-ray analysis of em- 

 bryonic processes," Jour. Exp. Zool., 85). 



Turning to the gradient concept, the following points are of interest in 

 relation to the problem of differentiation. The physiological gradients are 

 real, not hypothetical; they are activity gradients involving protoplasmic 

 dynamics as well as substrate; when they are altered, the course of differ- 

 entiation is altered (chaps, ii, v-vii). When they are obhterated, axiate 

 development and differentiation do not occur, even after the inhibiting 

 factors have been removed. New patterns can be initiated by external 

 differentials which affect metabolic rate (see chap. xi). A relation between 

 the gradient pattern and differentiation is evident, but there is still the 

 question whether this pattern provides an adequate basis for differentia- 

 tion. The differences distinguishable at different gradient-levels in early 

 stages appear to be primarily quantitative ; but what is quantitative and 

 what qualitative in an activity gradient in a living protoplasm is perhaps 

 a somewhat academic question. Assuming, however, that a gradient may 

 be primarily quantitative, at least as regards its dynamic characteristics, 

 can specific or quahtative differences originate at different levels of it? 

 Uptake of oxygen, intake and transformation of nutritive material already 

 present, breakdown of certain molecules, synthesis of others, and dis- 

 charge of CO. and other metaboUtes are factors of the metabolism of living 

 protoplasms. In a region of high rate of metabolism, transformation of 

 nutrition may occur as rapidly as it becomes available, in part perhaps 

 by complete oxidation, in part by partial breakdown, recombination, and 

 synthesis of new molecules. In a region of lower rate concentrations of 

 nutritive material and of oxygen available in relation to rate of transfor- 

 mation are undoubtedly different, and metabolism there may result in 



