SOME CONSIDERATIONS CONCERNING THE NATURE AND ORIGIN OF PHYSIOLOGICAL GRADIENTS

Abstract

The paper is a survey of the evidence bearing upon certain aspects of the problem of integration or pattern in organisms. The organism represents a definite pattern or integration of some kind. The pattern of the organism is of a higher order of magnitude than protoplasmic pattern since it involves masses of protoplasm or even cells each of which may possess the entire protoplasmic or cell pattern. Protoplasm apart from environment does not appear to possess any inherent mechanism for originating such pattern. While specific differences appear in the development and differentiation of pattern in each organism, in its more general features such as polarity and symmetry, this pattern appears to be largely independent of specific differences in protoplasmic constitution. This fact suggests that it represents primarily a non-specific or quantitative condition or relation in a specific protoplasm. Many different lines of evidence which are briefly reviewed indicate, first, that the simplest form of axiate pattern is primarily a gradient in physiological condition, involving the fundamental metabolic reactions as well as various other factors, and second, that such gradients arise in the final analysis from differential exposure to external factors which affect the rate of protoplasmic activity. The primary physiological relations established in this way are those of excitation and transmission but from the first moment of differentiation chemical or transportative correlation becomes possible and plays an increasingly important role in the progress of development.