130 



Essays in Biochemistry 



that agents which affect one species or even one cell type in a species 

 should exhibit parallel or similar effects in another species or another 

 cell type. Thus the application of these substances to E. coli must 

 be weighed as an illustration of general principles without expectancy 

 of utility in higher forms. 



OH 



\ 



OH 



"sN-^ JL^ /° H HjN-L^ JL 



/ 



HoN 



III 



NaOSO-CHo-N 



IV 



OH 



// H,N 



VI 



OCH 



O.N 



9 N 



OCH 



VII 



VIII 



IX 



Fig. 3. I. Guanine. II. 8-azaguanine. III. Acetylazaguanine. IV. Azaguanine 



sulfoxalate. V. 5-amino-7-hydroxypyrazolo-[4,3-d]-pyrimidine. VI. 6-amino-4- 



hydroxypyrazolo-l 3,4-d]-pyrimidine. VII. 4-methoxy-6-nitrobenzotriazole. VIII. 



6-hydroxy-4-nitrobenzimidazole. IX. 5-methoxy-7-nitroquinoxaline. 



The particular value of experimentation with the frog embryo, how- 

 ever, lies in the possibility of destruction between cleavage and dif- 

 ferentiation. The new compounds described above are still to be 

 explored in the light of their effects on energy cycles, but they have 

 already been found to exhibit stage specificity. 18 One group of com- 

 pounds, mostly quinoxalines, 5-methoxy-7-nitroquinoxaline, for exam- 

 ple, selectively inhibits and arrests embryos in early cleavage stages. 



