THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 269 



Dithiodiglycol (M/1,000), on the other hand, does not at all pre- 

 vent regeneration; it may even occasionally be faster than in the con- 

 trols. The mesenchyme of the regenerating tail is particularly baso- 

 philic, and mitotic activity is high. 



Finally, the eflPects of the SH-SS equilibrium have also been studied 

 on the regeneration of the head in planarians (Brachet, 1959, 1960). 

 Despite some difficulties, the following general conclusions can be 

 drawn. Mercaptoethanol (M/300 to M/1,000) again exerts a consider- 

 able inhibitory eflFect on the regeneration of planarians that have been 

 sectioned ahead of the pharynx. In most cases no formation of blastema 

 can be seen on the living organisms or on sections (Figures 21 and 22). 

 Once more, the inhibition of blastema formation is presumably due to 

 inability of the cells to migrate rather than to a block in mitotic 

 activity. 



Here too, dithiodiglycol is more toxic than mercaptoethanol; it 

 had to be used at concentrations of the order of M/3,000 to M/ 10,000. 

 Under these conditions, regeneration proceeds normally, except that the 

 pigmentation of the regenerated head is definitely slowed down, if not 

 completely inhibited. 



The results of these experiments oh the role of the thiol-disulfide 

 equilibrium in morphogenesis have been presented in some detail be- 

 cause they are too recent to allow us to draw any precise conclusion. It 

 is our hope that this presentation will encourage other workers inter- 

 ested in morphogenesis to test the effects of such useful reagents as 

 mercaptoethanol and dithiodiglycol on other biological systems than 

 the ones we studied. It certainly would be of importance to know 

 whether, as the present data suggest, the SH-SS equifibrium of the 

 surrounding medium (and probably the internal SH-SS equilibrium, 

 as a consequence) always is a factor of primary importance for 

 morphogenesis. 



One would also, of course, very much like to know how mercapto- 

 ethanol acts on the cell biochemically. This question is now under study 

 in our laboratory. Although it is too early to draw any conclusions, it 

 can be said that mercaptoethanol almost certainly does not exert its 

 inhibitory effects by any common sort of mechanism. It does not reduce 

 markedly the oxygen consumption or the ATP content of the cell; it 

 does not activate proteolytic enzymes which might counterbalance pro- 

 tein synthesis; it does not quickly inhibit the synthesis of nucleic acids 

 or proteins, as judged by autoradiography studies on the incorporation 

 of labeled precursors into these substances. 



To find out what it actually does remains a task for the future. But 

 it can be hoped fairly confidently that the reagent inhibits a rather spe- 

 cific biochemical process (perhaps the contractility of a fibrous pro- 

 tein ) which must be of major importance for morphogenesis in widely 

 different biological systems. 



