230 The Nature of Biological Diversity 



materials, how they are produced and how they operate. It seems 

 clear that essential components of the material are large-molecular. 

 This is suggested hy the restraining effect, already mentioned, of filter 

 porosities in the 0.1 micron range. Sieving in terms of molecular size 

 is not hy any means the only mechanism hy which the filter may re- 

 strain mobility, but it is noteworthy that 0.1 micron is quite large in 

 terms of molecular dimensions. Moreover, cellophane blocks the in- 

 ductive effect. This can be shown by simply interposing cellophane 



=L 



CO 

 CO 



o 



r- 



UJ 



> 



X 



o 



-z. 



UJ 

 CO 

 LlJ 



50 



45 h 



40 



35 



30 h 



25 



20 



15 



10 - 

 5 - 







TUBULES | NO TUBULES 



□ 



|o 



I 

 l i 



D 



J L 



O 



±AA, SINGLE LAYER 

 O TA, MULTIPLE LAYER 

 a TH, MULTIPLE LAYER 



J I L 



20 40 60 80 100 120 140 



FILTER THICKNESS ( \x ) 



PLATE V. FIG 11. Effect of separation distance (filter thickness) on metanephro- 

 genic mesenchyme thickness and tuhule formation. AA, TA, TH, filters of different 

 porosity and thickness. Each point represents an average of four or five cases. 



between the interactants, but it is shown more dramatically by making 

 use of the multiple-filter technique, as in the measurement of trans- 

 mission distance. If a three-layer assembly is used, the middle layer 

 of which consists of cellophane with a small hole (Fig. 12), a local- 

 ized induction is obtained only immediately over the hole (Fig. 13 \ . 

 This seems to say not only that the cellophane is "opaque" to the 

 activity, but that it comes through the small pinpoint hole to form 

 a "bright" spot on the opposite side. It is interesting that this 

 "shadowing" effect of cellophane is not only what would be expected 



