94 THE ORIGIN OF THE NERVOUS SYSTEM 



until in the higher animals and man it becomes almost 

 inconceivably complex. Moreover, from the moment 

 of its appearance as a factor in organismic pattern, 

 chemical correlation constitutes a factor in organismic 

 integration, which develops along different lines from 

 nervous integration and makes possible other kinds of 

 control and relation than those determined through 

 the nervous system. The internal secretions and all 

 so-called hormones of the higher animals and man con- 

 stitute the most advanced stages attained in this 

 development. 



Differentiation appears, not only in the development 

 of the mature organism from the egg, but in many animal 

 eggs the appearance of the yolk in the protoplasm con- 

 stitutes a characteristic differentiation. The relation of 

 this differentiation to the physiological gradient in the 

 egg is of interest. Very commonly a gradient in yolk 

 accumulation appears, the apical (" animal") pole of the 

 egg showing little or no yolk and the amount increasing 

 toward the basal (" vegetative ") pole. Yolk is sub- 

 stance which can under certain conditions be oxidized 

 and furnish energy, but it is evidently not oxidized to 

 any great extent during the growth period of the egg in 

 the region where it accumulates, and it has been sug- 

 gested that a reduction is involved in yolk formation. 

 Evidently the different regions of the egg are different 

 in some way, and the gradient in yolk accumulation 

 suggests that the difference is essentially quantitative, 

 not qualitative. Moreover, we find in such an egg a 

 quantitative gradient in physiological activity, and 

 when development begins the region containing least 

 yolk shows the greatest developmental and apparently 



