DIFFERENTIAL GROWTH I75 



shaped pattern. Cell movements and divisions will be directed at the 

 area, instead of around it as in the preceding example. Many embry- 

 onic invaginations (e.g. blastopore, neural tube, nasal pits) seem to 

 take their origin from such localized contractile centers in the surface 

 coat of the germ (24), 



Figure 2. Diagrams illustrating the effect of local contraction (top) and local ex- 

 pansion (bottom) on the structure of a reticular matrix. 



It was very illuminating to find that certain biochemical activities 

 attending growth and differentiation may lead to the same type of ex- 

 pansion and contraction patterns (45). The expansion type, for in- 

 stance, is observed around areas with proteolytic activity. When a local 

 process dissolves part of an erstwhile coherent protein network, the re- 

 maining portion, owing to its elastic tension, retracts from the hole and 

 assumes a pattern like that shown in Figure 2 Be, much like a pierced 

 cobweb. A contraction pattern such as in Figure 2 Ac, on the other hand, 



