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GASTRULATION 



{Top) Tritmus torosus, gastrula. (a) Surface view of the blastoporal region 

 after part of the surface layer has been removed, (b) Isolated bunch of flask 

 cells from the lateral blastoporus lip, containing larger endodermal and smaller 

 mesodermal cells. {Center) A. punctatum, gastrula. Surface view of the blasto- 



{Continued on facing page.) 



