234 The Nature of Biological Diversity 



(Fig. 17), when higher-porosity filters (about 0.5 micron) are used, 

 and declines very much more rapidly when lower-porosity (about 

 0.1 micron) niters are used. The impression is that there is quite good 

 agreement between the distribution of label and of inductive activity 

 with distance. 



It is interesting that the gradient of label exists not only in the 

 filter but in the surrounding clot in which the tissue is embedded 

 (Fig. 18). In fact, as one looks at the autoradiogram, one notes first 



-f * * ^ v. * ^* 



- • '<. . 



v ■.- «s * , - 



PLATE VIII. FIG. 16. Autoradiogram of transfilter culture involving dorsal spinal 

 cord (below), heavily labeled by exposure to tritiated amino acids. Note grains 

 above filter and over metanephrogenic mesenchyme on opposite side of filter. 



the dense and reasonably sharply bounded "picture" of the dorsal 

 cord, and then becomes aware — particularly knowing the result of the 

 grain counts — that there is a much less dense "halo" of grains grading 

 off around it. 



Recent data obtained by William Koch, a graduate student in our 

 laboratory, indicate that the label observed in the filter is noncello- 

 phane-passing. He has repeated the experiment referred to above 

 involving two filter layers with intervening cellophane. He finds label 

 in the filter on the dorsal cord side of the cellophane, but none in the 

 cellophane or in the filter on the opposite side. Under these circum- 



