June, 1921] 
CHILD PHYSIOLOGICAL CORRELATION 
287 
A very brief survey of certain lines of work is necessary by way of intro- 
duction to the experiments of which I wish particularly to speak. Study of 
several hundred species, including animals from all the chief groups and 
many algae and some other plants, have shown that physiological polarity 
and symmetry in their simplest terms consist of gradients in physiological 
condition and activity of the protoplasm or cells composing the organism. 
These gradients have been called axial, metabolic, or physiological gradients. 
That they have to do with the fundamental physiological condition of 
the protoplasm is clearly shown by the many different lines of evidence 
which demonstrate their existence. They appear as gradients in suscep- 
tibility to certain toxic ranges of concentration or intensity of external 
agents, e.g., cyanides, heavy metal salts, anesthetics, acids, alkalies, other 
neutral salts, CO2, various dyes, extremes of temperature, and the negative 
condition, lack of oxygen. Within certain limits of concentration or inten- 
sity these susceptibility gradients are non-specific, i.e., essentially identical 
in their larger features with all external agents tested, at least in the simpler 
animals and plants and in the earlier stages of development of higher forms. 
It has been shown that these differences in susceptibility are indicators of 
differences in rate of fundamental metabolism, particularly oxidation. The 
physiological gradients can be demonstrated as gradients in the rate of 
penetration of non-toxic or only slightly toxic vital dyes. Again, in dilute 
solutions of the oxidizing agent KMn04 they appear as gradients in rate and 
amount of reduction of the salt. In certain cases the indophenol reaction 
has been used, and a gradient in the rate of appearance of the indophenol 
suggests a gradient in oxidizing enzymes. A gradient in electric potential 
is a characteristic feature of the physiological gradient in all forms thus far 
examined, though in some plants the electrical situation is apparently com- 
plicated by the occurrence of reactions which give rise to opposite potential 
differences, viz., the oxidations and photosynthesis. And finally, in animals 
in which it has been found possible to determine the oxygen consumption 
and CO2 production of different regions along the axis, differences corres- 
ponding to those indicated by other methods have been found. It has not 
yet been possible to apply all these methods to each species examined. 
These physiological gradients also very commonly appear in differences in 
structure of the protoplasm along the axis, as in many plant and animal 
embryos, and they are definitely related to differences in rate of develop- 
ment and differentiation. 
It has also been possible to show that the localization and differentiation 
of organs and parts occur in a definite relation to the physiological gradient, 
in fact are determined by it. The most active region as finally determined, 
i.e., the region of highest susceptibility, of greatest permeability, of greatest 
reducing capacity, of highest external electro-negativity, and of highest 
rate of respiration in the polar axial gradient, becomes the apical end of 
the axis, or in animals the head, and the other organs develop at different 
