262 CELLS, TISSUES, AND ORGANISMS 



surface-coat material, whose embryological importance has just been 

 discussed, presents a very high elasticity, characteristic of many fibrous 

 macromolecules. 



The importance of -SH-containing substances for morphogenesis 

 in sea-urchin eggs has also been clearly shown by Runnstrom and 

 Kriszat (1952), Lallier (1951), and Biickstrom (1958, 1959). As Back- 

 strom pointed out, "the -SH metabolism seems to play an important 

 role in the process of animalization and probably also in the antagoniz- 

 ing process of vegetalization." 



In the case of amphibian eggs, the effects on morphogenesis of a 

 number of "classical" sulfhydril reagents ( mono-iodacetic acid, mono- 

 iodacetamide, chloropicrine, chloracetophenone, oxidized glutathione, 

 arsenite, etc.) have been studied by a number of authors (Brachet, 

 1944; Beatty, 1949; Rapkine and Brachet, 1951; LalHer, 1951; Barth, 

 1956; Deuchar, 1957; ten Gate, 1957; etc.). All these agents produce 

 similar effects. The nervous system remains a thick, open plate, while 

 the differentiation of chorda and somites are relatively normal. 



More recently the effects on amphibian morphogenesis of new 

 sulfhydril reagents, introduced in biological research by Mazia ( 1958, 

 a, b) in his important studies on mitosis in sea-urchin eggs, have been 

 investigated. These agents are ^-mercaptoethanol (HSCH2-CH2OH), 

 which is strongly reducing, penetrates easily into living cells and is rela- 

 tively non-toxic, and its oxidized counterpart, dithiodiglycol ( HOCH2- 

 CH2-S-S-CH2-CH2OH), which easily oxidizes -SH groups of proteins. 

 Mercaptoethanol (Brachet, 1958a; Brachet and Delange-Cornil, 1959; 

 Seilern-Aspang, 1959) exerts powerful inhibitory effects on morpho- 

 genetic movements during gastrulation and neurulation; it inhibits, in 

 an almost specific way, the closure of the neural plate (Figure 15). 

 Dithiodiglycol, at relatively high concentrations (M/3,000), also in- 

 hibits the closure of the neural plate (Brachet and Delange-Cornil, 

 1959), but in a different way: the medullary plates formed in the 

 presence of mercaptoethanol are thin, while those formed in embryos 

 treated with dithiodiglycol are exaggeratedly thickened (Figure 16). 

 Generally speaking, dithiodiglycol ( which, at these concentrations pre- 

 sumably acts by oxidizing thiol groups in the egg proteins), behaves 

 very much like the "classical" sulfhydril reagents, such as iodo- 

 acetamide, chloropicrine, or arsenite. 



It is interesting to note that mercaptoethanol also exerts strong in- 

 hibitory effects on morphogenesis in the unicellular alga Acetabiilaria 

 mediterranea (Brachet, 1958b). At M/300 concentration it completely 

 inhibits "cap formation" in entire algae or non-nuclear fragments, but 

 without interfering with growth or production of sterile whorls. 



The fact that mercaptoethanol inhibits morphogenesis in biological 

 systems as different as amphibian eggs and Acetahularia suggests the 



