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TEXT-BOOK OF EMBRYOLOGY. 



is limited mainly by protentoderm, the "completion cavity" by yolk entoderm. 

 The breaking of the partition between the invagination cavity and the "com- 

 pletion cavity" results in the formation of the archenteron proper or primitive 

 gut, which is thus lined partly by protentoderm and partly by yolk ento- 



Ectoderm 



Blastopore 



Yolk plug 



Fig. 39. — Successive stages of gastrulation in the frog, showing especially the formation of the 

 protentoderm, yolk entoderm and "completion cavity." Schultze, Bonnet. Com.pl., "Completion 

 plate." 



derm, the two being from now on called simply entoderm. The somewhat 

 thickened area of yolk cells at the junction of the protentoderm and yolk 

 entoderm is designated by Bonnet, the " Erganzungsplatte " or "completion 

 plate" (Fig. 39, d, e). 



