THEORIES OF INDIVIDUALITY 35 



action of an external factor on a morphologically and 

 physiologically homogeneous living mass. 



The formation of metabolic gradients in another way 

 is possible, at least in single cells. If, for example, inac- 

 tive substances of different weight from the active pro- 

 toplasm are present in the cell, and if the position of 

 the cell with respect to the force of gravity remains 

 unchanged for a sufficiently long time, the inactive 

 substances and active protoplasm may be more or less 

 definitely localized in different parts of the cell and so a 

 metabolic gradient may result. Again, it is conceivable 

 that continued intake of nutrition at some particular 

 point of the cell surface might load that portion of the 

 cell with inactive reserve substances and so give rise 

 to a gradient. To what extent the origin of metabolic 

 gradients is due to such factors as this is still a question. 

 In the frog's egg gravity undoubtedly contributes to in- 

 tensify the existing gradient by bringing about a further 

 separation of the heavier yolk granules and the lighter 

 protoplasm, but it is not responsible for the origin of 

 the gradient. Unquestionably the primary factor in 

 the origin of these persistent metabolic-protoplasmic 

 gradients is in most cases at least a reaction-gradient, 

 and the persistent or permanent gradient in the proto- 

 plasmic substratum is secondary. 



Such metabolic gradients are, I believe, the simplest 

 expression of physiological unity and order in living 

 protoplasm, and at the same time they are the simplest 

 and primary form of the organic axes of so-called 

 polarity and symmetry and the starting-point of the 

 mysterious ^'organization." They are factors in deter- 

 mining the direction of growth and differentiation and 



