/ 



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V- 



4 ' ;4r^ 



'i 



. • • ^^ 



•A 





-yi^' 



Figure 12. Formation of atypical ectoderm in an amphibian egg treated with 

 ribonuclease at the morula stage (Brachet and Ledoux, 1955) . 



Figure 13. Formation of a nervous system in an amphibian egg treated with 

 a mixture of RNA and ribonuclease at the morula stage (Brachet and Le- 

 doux, 1955). 



