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 
