236 The Nature of Biological Diversity 



inductive activity. Labeled metanephrogenic mesenchyme, for ex- 

 ample, puts label into the filter and across it into unlabeled dorsal 

 cord, though no knotvn inductive activity moves in this direction. 

 Moreover, label is present in the filter after incubation times as short 



Counts 



240 



200 



160 



120 



80 



40 



Clot Gradient 



L5_ x_ ^kKgrpupi. 

 Background 



1 I I I 



20 40 60 80 100 120 



Distance from source ( _u ) 



PLATE X. FIG. 18. Curve of grain counts with increasing distance in clot sur- 

 rounding dorsal spinal cord labeled with tritiated amino acids. 



as 3 hours, but definitive changes in the metanephrogenic mesenchyme 

 are not known to occur until 24 to 30 hours. Keeping these reserva- 

 tions in mind, it is safe only to say that inductive activity appears to 

 be associated with a kind of "halo" of large-molecular material, in- 

 cluding protein, which exists in the immediate vicinity of dorsal 

 spinal cord. 



