AXIAL GRADIENTS, a term ap plied to the regular decrease in the rate of protoplasmic activities, which is ob served in organisms between one point of the body and another. The region where the rate is highest is the head-end of animals, and the growing point of plants ; and from these points, the grad ual decrease in rate of activities follows the axis of the organism. It is for this reason that the gradients are called "axial." The demonstration of axial gradients in the higher animals is made difficult by the high degree of specializa tion which they show ; but it has been established that they exist in Protozoa, Coelentera, Platyhelmia, Annelida, lar vae of Echinodermata, embryos of fish, tadpoles of frogs and embryos of birds. In addition, axial gradients have been demonstrated in a number of eggs. The methods of demonstration make use of the fact that the rate of activities of the protoplasm can be roughly measured by the rate of oxidation. The methods are the following : (I) direct susceptibility to toxic substances; (2) indirect suscep tibility to toxic substances through accli matization; (3) differential reduction of potassium permanganate to manganese dioxide in the tissues; (4) measurement of the output of carbon dioxide; (5) detection of difference of electric potential.
The importance of axial gradients lies in the fact that they are set up in previously homogeneous protoplasm by the action of external stimuli, and that, once set up, they constitute the prime differentiation of the organism during development, including especially polarity and symmetry. The relative rates of activity at different levels on the gradients determine the quality of the tissue which in development will differentiate at those levels. (See EXPERIMENTAL EMBRYOLOGY.) BIBLIOGRAPHY.--Nearly all the important literature on axial gradiBibliography.--Nearly all the important literature on axial gradi- ents is contained or referred to in C. M. Child, Physiological Founda tions of Behavior (1924). (G. R. DE B.)