INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 447 



them to enter into combination with substances at the surface of other 

 neighbouring cells. He has laid particular stress on the influence of such 

 reactions in changing the constitution of the internal cytoplasm by anchor- 

 ing certain constituents on the surface, and he suggests that this may 

 provide a general mechanism of differentiation (Fig.19.3), It is not easy 

 to admit the general importance of the hypothesis in connection with the 

 differentiation of substance, if only because differentiation can proceed 

 quite well in unicellular Protozoa, or in completely isolated amphibian 

 notochord cells (Mookerjee 1953), or pigment-forming cells (Twitty and 

 Niu 1954). But processes of this kind may play a major part in morpho- 

 genesis. Schmitt (1940) has also discussed, in what one might call general- 

 ised chemical terms, how cell surfaces might be bound together by a 

 an intermediate layer which could react with both of them (Fig. 20.7), 



fl^-Vc' 





««n 



H,o o s-^ 





& 



Histone 



Zippei 

 action 



Figure 20.17 

 A represents a rather flat cuboidal cell; its surface where it makes contact 

 with the next cell is highly solvated, and adhesion between the cells is slight. 

 If a desolvating agent (calcium ions, or histone) is introduced, the cells will 

 be drawn together as though by a zipp fastener, eventually (C) forming tall 

 cells with considerably greater surface of mutual contact. (From Schmitt 



1941.) 



