PHYSIOLOGICAL GRADIENTS 95 



the greatest oxidative activity, and is certainly doing 

 the most work. Evidently the localization of the yolk 

 in the egg is determined by the axial gradient and con- 

 stitutes the first differentiation in relation to that 

 gradient. The facts indicate very clearly that the yolk 

 accumulates in the region of least oxidation, even 

 though the substances from which it is formed enter at 

 the opposite pole, as they do in many cases (Fig. 5). 

 Such differentiation along the axis of the egg becomes 

 the basis of the two primary germ layers, the ectoderm 

 arising from the more active, the entoderm from the less 

 active, region. The region of origin of mesoderm differs 

 in different forms, but is often intermediate between 

 ectoderm and entoderm. The gradient hypothesis 

 maintains that differences in behavior of the different 

 egg regions and of the germ layers arising from them 

 result from the fact that they represent different levels 

 of the gradient. 



The gradient which determines the differentiation of 

 the oogonial cytoplasm into yolk-bearing and yolkless 

 regions is merely the beginning of a more or less complex 

 series of events, each of which is determined in a definite 

 and orderly way by the preceding. It will be evident 

 at once that as soon as any such differentiation along a 

 gradient occurs chemical or transportative correlation 

 becomes possible, and it is certain that such correlation 

 plays an increasingly important and complex role as 

 development progresses. The accumulation of yolk in 

 one region of the egg becomes itself a basis for trans- 

 portative correlation, for most of this yolk is finally used, 

 not in the region where it accumulated, but in the 

 region which originally contained little or no yolk. 



