June, 1921] CHILD — PHYSIOLOGICAL CORRELATION 289 
whether the physiological gradient shows any similarity to an excitation 
gradient. All living protoplasm is excitable and to some degree capable of 
transmitting excitation. Excitation in its most primitive form appears 
to be an acceleration in the rate of living, particularly as regards the energy- 
liberating aspects of life. Where specialized conducting paths are not 
present transmission of excitation occurs with a decrement, that is, the 
transmitted change becomes weaker and finally disappears at a greater or 
less distance from the point of origin. Such a process of excitation and 
transmission gives rise to an excitation-transmission gradient. We usually 
think of such gradients as temporary or reversible, but the physiological 
gradients show all the characteristics of excitation gradients which have 
become more or less permanent. 
Moreover, it has been shown experimentally that these gradients can 
be produced in cells or cell masses by subjecting them to a quantitative 
differential in the action of external factors. For example, new gradients 
and so new polarities can be determined experimentally in the simpler 
animals by a sufficient difference in oxygen supply, by an electric differen- 
tial, perhaps in some forms by a light differential, and probably also by 
various other differentials. A differential inhibition may have the same 
effect as a differential excitation or acceleration. Turning to the plants, 
the polarity of the Fucus egg, which is primarily a gradient, is determined 
by the differential action of light, and the polarity of the Equisetum spore 
has a similar origin. The relation of dorsiventrality and symmetry to 
light in the plants is a familiar fact. In order to establish a gradient suffi- 
ciently persistent to serve as a physiological axis, the differential action of 
the external factor must persist for a certain length of time dependent on 
the nature and intensity of the factor and the character of the protoplasm. 
The fixation of such a gradient in protoplasm must depend upon the occur- 
rence at the different levels of changes which are more or less irreversible 
under the existing conditions and which differ in degree according to level. 
Once established in a cell or cell mass, such a gradient may persist through 
division or other reproductive processes and become the basis of the axis 
of the new individual or individuals. In other cases the original gradient 
may disappear in reproduction and a new gradient arise. In many eggs, 
both animal and plant, the gradient is apparently determined by a differen- 
tial in relation to the parent body, such, for example, as the difference in 
conditions at the attached and the free end of the egg; but in some eggs it 
may perhaps persist from earlier cell generations, while in others, as in 
Fucus, it is determined by a factor external to the organism. 
We come now to the question of the nature of the dominance or control, 
and this involves the question of the nature of transmission. Many 
hypotheses have been advanced concerning the process of transmission, 
and most of them connect it in one way or another with the electric changes 
which are a characteristic feature of excitation. On the basis of extensive 
