METABOLIC GRADIENTS 65 



cells where the rate of oxidation is highest the precipitate 

 is formed most rapidly and vice versa. In the starfish 

 embryos and early larvae the precipitate appears first in 

 the cells of the apical region, and a very definite color 

 gradient along the main axis arises in living animals, 

 while in animals which have been killed before staining no 

 gradient appears. This method is undoubtedly capable 

 of wide application. 



These various methods and results indicate the 

 possibilities of demonstrating the existence of the meta- 

 boHc gradients in organisms by biochemical and physio- 

 logical methods. Unquestionably future investigation 

 will give us much more accurate and extensive data than 

 we possess at present. 



EMBRYOLOGICAL EVIDENCE FOR THE EXISTENCE OF 

 AXIAL METABOLIC GRADIENTS 



Gradients in rate of cell division, size of cells, con- 

 dition or amount of protoplasm in the cells, rate of 

 growth, and rate and sequence of differentiation are 

 very characteristic features of both animal and plant 

 development. Such gradients are definitely related to 

 the axes of the individual or its parts, and are evidently 

 expressions of axial metabolic gradients. While the 

 existence of such gradients indicates the existence of 

 gradients in activity of some sort, the various kinds 

 of gradients are not all necessarily present where meta- 

 bolic gradients exist. In some cases the visible gradient 

 may be a gradient in rate of growth or in protoplasmic 

 constitution; in still others a gradient in sequence of 

 differentiation, etc., and sometimes metabolic gradients 

 exist without any structural indications of their presence. 



