266 CELLS, TISSUES, AND ORGANISMS 



Stimulating cap production and inhibiting the formation of the sterile 

 whorls ( Figure 18 ) . The two processes are obviously antagonistic, and 

 the outcome seems to be regulated, among other things, by the 

 sulfhydril-disulfide equihbrium. 



In order to test further this view that the sulfhydril-disulfide equi- 

 librium regulates a morphogenetic process ( cap or whorls formation ) , 

 the effects of some of the "classical" sulfhydril reagents on the regen- 

 eration of nucleate and anucleate Acetabularia fragments have been 

 studied. Two of them, p-chloromercuribenzoate ( 10 '''M ) and p-iodoso- 

 benzoic acid ( lO'^M ) exerted definitely favorable 'effects on cap forma- 

 tion in anucleate fragments, but iodoacetamide did not. These observa- 

 tions thus confirm the view that an excess of thiol groups is detrimental 

 to the production of caps, and that a decrease of these groups is favor- 

 able for morphogenesis ( as in amphibian embryos ) . 



Similar observations can be made in the case of the regeneration 

 of the tail of tadpoles ( Brachet, 1959, 1960 ) ; it is completely inhibited 

 by mercaptoethanol M/300 (Figures 19 and 20). Four days after the 

 section, there is almost no regenerating blastema and the tadpoles are 

 still unable to swim properly. Since mitoses are far from absent, the 

 inhibition of blastema formation is probably due to a reduction of cell 

 migration and, possibly, to an incapacity of the sectioned chorda to 

 elongate normally in the presence of mercaptoethanol. 



Figure 18. Same experiment as in figure 17, but treatment with M/10,000 

 dithiodiglycol. These fragments show good cap formation. 



