EMBRYONIC INDUCTORS AND ORGANIZERS 



453 



The regions mapped are not, in general, coextensive with embryonic fields. 

 Usually they are more limited than the fields, that is, they show the 

 regions which will actually develop into certain organs, not the regions in 

 which development of those organs is possible. Physiologically, such a 

 map may represent nothing more than a quantitative gradient pattern, 



Fig. 154, A-D. — Cell migrations in chick embryo at four stages, according to Pasteels; 

 arrows drawn in unbroken line indicate movements involving both cell layers; those in broken 

 line, movements of surface layer only (after Pasteels, 1936&). 



the developmental fate of the regions being determined by their position 

 in the gradient system. In other words, the regional maps of early stages 

 do not represent actual patterns but are projections of the patterns of 

 later stages back onto a stage in which the actual pattern is simpler and 

 more general in character. It is not necessary to think of the various parts 

 of the embryo as spread out on the blastula or blastoderm, as the maps 



