PHYSICAL PROPERTIES OF THE INDUCING AGENT 85 



no reason why it should not be considered as a very possible ex- 

 planation in the case of the primary organizer as well. 



From the embryological viewpoint, the intercellular matrix of 

 Grobstein (1955, 1956) is a development of Holtfreter's (1943) sur- 

 face coat. This matrix is a material which is soluble in alkaline 

 media and which presents a marked elasticity. It is present in the 

 cell cortex, in the fertilized egg, and it holds the cells together 

 thus acting as an intercellular cement. The surface coat becomes 

 reinforced in the dorsal lip at the time of gastrulation. It is still 

 further developed in the neural plate at the neurula stage. Dis- 

 solution of the surface coat by weak alkalis (e.g. potassium cyanide) 

 results in the separation of the cells which form the embryo. This 

 explains why a treatment of gastrulae with relatively concentrated 

 cyanide solutions produces a dissociation of the cells first at the 

 animal pole, and then in the dorsal lip. The dissociated cells soon 

 undergo cytolysis. 



Very little is known, unfortunately, about the chemical nature of 

 the surface coat or the intercellular matrix. Treatment with the 

 calcium-complexing agent versene (EDTA) produces separation 

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

 is greatly enhanced by the addition of small amounts of ribonu- 

 clease (Brachet, 1959). It is also known that proteolytic enzymes, 

 e.g. trypsin, readily 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 is of 

 a ribonucleoprotein nature, although cytolysis of part of the cells 

 would easily explain the results obtained on dissociated cells by 

 Curtis (1958). Cytochemical studies are, however, in favour of 

 Curtis' conclusion that the intercellular cement is a ribonucleo- 

 protein. In amphibian eggs and embryos, cell membranes give very 

 strong reactions for RNA. Pending further experimental and more 

 precise work, it seems safe to conclude that the intercellular matrix 

 is made of a ribonucleoprotein associated with calcium ions and, 

 possibly, mucopolysaccharides. If RNA is really involved in the 

 intercellular matrix composition, its role in induction would be- 

 come more probable and easier to understand. 



References p. 90/93 



