Microenvironmental Influences in Cytodifferentiation 235 



stances label is found in the metanephrogenic mesenchyme hut at 

 reduced levels in comparison with noncellophane cultures. This label 

 presumably represents nonfixable small molecular material which 

 passes cellophane. 



Counts 



200 



HA Filter 



-2-JL JL _Bac.k£rqu_nd_ 



Distance from source ( jj ) 



PLATE IX. FIG. 17. Curve of grain counts across thick filter with increasing 

 distance from spinal cord source labeled with tritiated amino acids. 



The entire body of data can be explained on the assumption that 

 dorsal spinal cord produces at free surfaces a protein-containing mate- 

 rial which moves into the environment over a limited distance. Among 

 its components this material includes one or more which is inductively 

 active. It is important to note that though inductive activity conforms 

 with the distribution of label, labeling does not necessarily indicate 



