270 CELLS, TISSUES, AND ORGANISMS 



Summary 



The following points have been presented and discussed in the 

 present paper: 



1. The role of ribonucleic acid, ribonucleoproteins, and microsomes 

 in the induction of the nervous system has been examined in the light of 

 grafting and explantation experiments. The relative role of ribonucleo- 

 proteins and proteins in archencephalic (neural) and spinocaudal 

 ( mesodermic ) inductive processes has also been discussed. 



2. The existence of gradients in ribonucleic acid and protein dis- 

 tribution and synthesis has been demonstrated. 



3. The results of experimental modifications of ribonucleic acid 

 synthesis on morphogenesis, and their eflFects on the ribonucleic acid 

 gradients in amphibian eggs, have been discussed in some detail. In 

 particular, the eflFects on development of chemical analogues of purines 

 and pyrimidines, of specific inhibitors of oxidative phosphorylations, of 

 centrifugation at various developmental stages, of heat shocks, of sub- 

 lethal cytolysis, of lethal hybridization, and of ribonuclease have been 

 examined. 



4. Finally, another important factor in morphogenesis, namely the 

 role of -SH-containing compounds, has been stvidied. The eflFects of 

 various -SH and -SS- reagents on morphogenetic processes in materials 

 as diflFerent as amphibian eggs, regenerating tadpoles, planarians, and 

 the unicellular alga Acetabularia mediterranea have been presented. 

 The importance of a thiol-disulfide equilibrium in morphogenetic proc- 

 esses such as neural-plate formation, pigment-cells diflFerentiation, "cap" 

 production in Acetabularia, and head or tail regeneration has been 

 emphasized. 



References 



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 Backstrom, S., 1939. "Studies on Sulfhydril-Containing Substances in Sea Urchin 



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