228 The Nature of Biological Diversity 



PLATE III. FIG. 7. Dorsal spinal cord in metanephrogenic mesenchyme. Note 

 induced tubules surrounding cord. 



against intervening distance, an "inductive activity" curve was ob- 

 tained (Fig. 11). This declined sharply beyond about 30 microns and 

 reached the level of nondetectability, by the criteria used, at 60 to 80 

 microns. The close association of induced structure to inductive source 

 thus appeared not to be based on a requirement for surface-to-surface 

 contact, but on limited transmission distance in active form of the 

 inductive material. Stated in another way, we seem to be dealing with 

 materials which are not of the cell surface itself, but which normally 

 are closely associated with it. In turn, this would imply that such 

 surface-associated, microenvironmental materials are important in 

 controlling differentiative processes. 



We would, of course, like to know very much more about these 



PLATE IV. Transfilter induction by dorsal spinal cord of epithelial tubules in 

 metanephrogenic mesenchyme. FIGS. 8, 9. Positive effect with two filter layers. 

 FIG. 10. Absence of effect with four filter layers. 



