PHYSIOLOGICAL BASIS OF PATTERN 27 



already be interpreted in physiological terms, and 

 usually the pattern even in the fully developed organism 

 shows a very definite relation to the primary gradients. 

 The evidence for the existence of metabolic or 

 physiological axial gradients is varied and extensive 

 and only a brief summary is possible here. The evi- 

 dence, as it stood five years ago, was briefly discussed 

 in the book Individuality in Organisms, but since that 

 book was written other methods of investigation have 

 been developed and many new data obtained, so that a 

 review of the subject is necessary at this time. 



THE EVIDENCE FROM STRUCTURE AND DEVELOPMENT 



First of all many eggs and embryos show an apico- 

 basal gradient in protoplasmic structure and content, 

 e.g., the gradient in yolk accumulation in many animal 

 eggs and embryos, and the gradient in protoplasmic 

 density and vacuolation in many plant embryos, and in 

 the vegetative axes of many of the simpler plants. A 

 gradient in the rate of cell division, growth, and differ- 

 entiation in relation first of all to the primary of apico- 

 basal axis, later in relation to other axes, is a very 

 general feature of at least the earlier stages of develop- 

 ment. This gradient appears first in the rate of cell 

 division and size of cells along the apico-basal axis in a 

 large proportion of both animal and plant eggs and in 

 many plant axes, as a gradient in rate of division and 

 cell size from the growing tip basipetally. It also 

 appears in the progress of morphogenesis and differen- 

 tiation along the axes, particularly in animals, although 

 in plants, except for the fact that the growing tip itself 

 remains embryonic, the course of differentiation is also 



