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 
