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The Nature of Biological Diversity 



emphasized by measurements made on the transmission distance of 

 the inductive activity, even using higher-porosity filters which allow 

 relatively free entry of cytoplasmic processes into the interspace. 

 Measurements were performed by interposing either single filters of 

 different thickness or multiple layers of filters of the same thickness 

 (Figs. 9 and 10). When the response, expressed either as thickness of 

 the metanephrogenic layer or as occurrence of tubules, was plotted 



£*l&i^&S(S!S 



PLATE I. Developing metanephric rudiment of the 11-day mouse embryo in vitro. 

 (X38.4.) FIG. 1. Immediately after explantation. FIG. 2. On the second day. FIG. 3. 

 On the fourth day. FIG. 4. On the seventh day. 



