PRIMITIVE PLATE 



Fig. 174. Formation of hypoblast (entoderm) layer in certain reptiles; major pre- 

 sumptive organ-forming areas of reptilian blastoderm. (A) Section through blastoderm 

 of the turtle, Clemmys leprosa. This section passes through the primitive plate in the 

 region where the entoderm cells are rapidly budded off (invaginated?) from the surface 

 layer. It presumably passes through (E) in the area marked entoblast. It is difficult to 

 determine whether the entoderm cells are actually invaginated, according to the view of 

 Pasteels, or whether this area represents a region where cells are delaminated or budded 

 off in a rapid fashion frorh the overlying cells. (B) Similar to (A), diagrammatized 

 to show hypoblast cells in black. (C) Section through early blastoderm of the gecko, 

 Platydactylus. Epiblast cells are shown above, primitive entoderm cells below. (D) A 

 later stage showing primitive plate area with the appearance of a delamination or prolif- 

 eration of entoderm (hypoblast) cells from the upper layer of cells. (E) Presumptive, 

 organ-forming areas of the turtle, Clemmys leprosa, before gastrulation. (F) Presump- 

 tive, organ-forming areas of the epiblast of turtle and other reptiles if the hypoblast is 

 budded off or separated from the underside of the epiblast without invagination. It is to 

 be observed that B and D represent modifications by the author. 



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