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GASTRULATION 



cells, arranged more or less in a sphere, each cell having essentially the 

 same potentialities as any other cell, there is now a mosaic of cell 

 groups of integrated differences. The differences in the various areas 

 may not be apparent physically, but they can be demonstrated func- 

 tionally and constitute the process of differentiation. This is probably 

 the most critical change that is brought about by gastrulation. 



Another reason for the critical nature of gastrulation lies in the 

 mechanics of the process itself. There is a localized interruption in the 



Gastrulation in the frog, Rana pipiens (photographs). (A) Initial involution 

 at the dorsal lip. (B) Crescent-shaped lips of the blastopore extending laterally 

 along the margin of the gerrn wall. (C) Half-moon-shaped involuted dorsal and 

 lateral lips of the blastopore. Presumptive areas or organ anlagen indicated. 

 (D) Lips of the blastopore completely encircle the exposed yolk plug. Rotation 

 of the entire embryo through 90°, 



