264 CELLS, TISSUES, AND ORGANISMS 



hypothesis that a biochemical system involving -SH and -SS- groups 

 may play a key role in morphogenetic processes. In order to test this 

 hypothesis, we have compared the eflFects of mercaptoethanol and 

 dithiodiglycol on a number of biological systems— amphibian eggs, 

 nucleate and anucleate fragments of Acetahularia mediterranea, regen- 

 eration of the tail of tadpoles, and regeneration of the head in pla- 

 narians ( Brachet, 1960 ) . 



As already mentioned, the most conspicuous result obtained when 

 amphibian gastrulae or neurulae are treated with mercaptoethanol 

 (M/lOO to M/300) is the complete cessation of morphogenetic move- 

 ments. However, mercaptoethanol is relatively non-toxic, especially 

 during neurulation (see Figure 15), and the blocked embryos survive 

 for several days. Experiments in which mercaptoethanol- treated organ- 

 izers were put together with normal ectoderm fragments and vice 

 versa disclosed the fact that mercaptoethanol acts more effectively on 

 the competence of the ectoderm than on the inducing power of the 

 organizer. These observations stand in good agreement with those 

 made on whole embryos, in which the neural plate fails to form or 

 close while chorda and somites differentiate relatively well (Brachet, 

 1958a, 1960; Brachet and Delange-Cornil, 1959). 



At lower concentrations of mercaptoethanol ( M/300 or M/1,000, 

 for instance) development proceeds further; after three or four days of 

 continuous treatment, one obtains strongly microcephalic embryos 

 which are more or less delayed in their development. A striking feature 

 of these embryos is an almost complete absence of melanophores and a 

 complete lack of pigmentation of the retina. Mercaptoethanol, prob- 

 ably by virtue of its reducing properties, thus exerts a profound inhibi- 

 tion of pigment formation, in the eye as well as in the skin. The results 

 in the case of Xenopiis are especially striking: the tadpoles that form 

 in the presence of mercaptoethanol have blue eyes. Obviously mer- 

 captoethanol inhibits the formation of melanin in a fairly specific way. 



The strong toxicity of dithiodiglycol, as compared to mercapto- 

 ethanol, is especially remarkable in the case of Pleurodeles neurula 

 stages. In a M/10,000 solution, cytolysis may occur within 24 hours. 

 But, in contrast to mercaptoethanol, dithiodiglycol does not markedly 

 delay or modify development before it exerts its lytic effects. At lower 

 concentrations (M/30,000 to M/100,000), dithiodiglycol has, if any- 

 thing, a slightly stimulating effect; in particular, the head sometimes 

 shows overdevelopment, and the tail may be longer than in the con- 

 trols. The tadpoles always show a higher degree of motility than the 

 controls. There is thus no doubt that mercaptoethanol and dithiodigly- 

 col exert opposite ejects in all respects. The absence of mitoses, the 

 microcephaly, the elongation of the body, which are characteristic of 

 the embryos treated with mercaptoethanol, are replaced after treatment 



