THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 261 



surface-coat material may very well be of paramount influence for egg 

 development. For instance, Curtis succeeded in adding surface-coat ma- 

 terial on the ventral side of amphibian eggs and obtained, as a result, 

 the formation of double embryos. In other words, the surface-coat 

 material behaves exactly as the "grey crescent" of experimental em- 

 bryologists, and it must play a most important role in morphogenesis. 



\"ery little is known, unfortunately, about the chemical nature of 

 the surface coat or the intracellular matrix. Treatment with the calcium 

 complexing agent versene (EDTA) produces the separation of the 

 gastrula cells. This effect of versene, as we have seen earlier, is greatly 

 enhanced by the addition of small amounts of ribonuclease (Brachet, 

 1959), and we have seen that this enzyme easily penetrates into the 

 eggs during cleavage (Brachet and Ledoux, 1955). It is also known 

 that proteolytic enzymes (trypsin, for instance) easily dissociate the 

 cells of the amphibian gastrula. After dissociation by various means, a 

 ribonucleoprotein is liberated ( Curtis, 1958 ) ; this fact suggests that the 

 intercellular matrix (surface coat) is of a ribonucleoprotein nature, 

 although cytolysis of part of the cells would easily explain the results 

 obtained on dissociated cells by Curtis. Cytochemical studies, how- 

 ever, favor Curtis' conclusion that the intercellular cement is a ribo- 

 nucleoprotein: in amphibian eggs and embryos, cell membranes give 

 very strong reactions for RNA. Pending further, more precise work, it 

 seems safe to conclude that the intercellular matrix is made of a ribo- 

 nucleoprotein, associated with calcium ions and, possibly, mucopoly- 

 saccharides. If RNA is really involved in the composition of the inter- 

 cellular matrix, its role in induction becomes still more probable and 

 easier to understand. 



Obviously the next task for the chemical embryologists will be 

 to isolate and try to identify the chemical nature of the surface coat 

 material. 



The role of sulfhtjdril groups 



The importance of sulfhydril (thiol, or -SH) groups for morpho- 

 genesis has often been emphasized. In the amphibians, for instance, it 

 has been suggested (Brachet, 1944; Rapkine and Brachet, 1951) that 

 the morphogenetic movements characteristic of gastrulation and neu- 

 rulation are closely linked to the reversible denaturation of fibrous, 

 myosin-like proteins. Oxidation of -SH into -SS- groups would trans- 

 form the fibrous molecule into its globular form; as a result, the shape 

 of the cells would change. That such fibrous macromolecules really 

 exist in amphibian eggs has been clearly demonstrated by Lawrence 

 et al. ( 1944 ) and by Ranzi ( 1955, 1957 ) . It is of special interest that the 



