232 The Nature of Biological Diversity 



of large molecules, but it suggests that mobility of the material is 

 primarily vertical in the filter substance with little lateral component. 

 This is in conformity with what is believed by the manufacturers to 

 be the structure of the filters used. It may be an important character- 

 istic for obtaining the transfilter inductive effect, since it would tend 

 to reduce dilution of materials in the interspace by the general en- 

 vironment. 



It also seems clear that protein is a constituent of the interspace 

 material. At an early stage in the study of the transfilter effect, it was 

 noted that the filter interspace between the tissues in sections differed 



PLATE VII. Residual spot in filter. FIG. 14. Before exposure to trypsin. FIG. 15. 

 After exposure to trypsin. 



slightly in staining and optical properties from the filter lateral to 

 the tissues. This led to studies of spinal cord incubated on filters in 

 the absence of metanephrogenic mesenchyme, in an effort to detect 

 inductive activity or material "secreted" by the spinal cord into the 

 filter. Such preparations, after incubation for 20 to 24 hours, were 

 fixed in alcohol-formalin. The spinal cord was found to be tightly 

 cemented to the filter, so tightly that it could be removed only by 

 scraping and fragmentation. When virtually all of the tissue had been 

 scraped away, a translucent spot was left, clearly different from the 

 surrounding filter ( Fig. 14 ) . The spot stained occasionally but vari- 

 ably with the periodic acid-Schiff procedure. When spotted filters 



