362 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



accurate diagrams of the reptilian blastoderm.) Surrounding the primitive 

 plate, the central blastoderm is thinner and is but one (occasionally two cells) 

 cell in thickness (see margins of figs. 174A, C). As development proceeds, 

 a layer of cells appears to be delaminated or proliferated off from the under- 

 surface of the primitive plate area (fig. 174C, D). This delamination gives 

 origin to a second layer of cells, the entoderm or hypoblast (Peter, '34). 

 Some of these entodermal cells may arise by delamination from more pe- 

 ripheral areas of the central blastoderm outside the primitive plate area. In 

 the case of the turtle, Clemmys leprosa, Pasteels ('37a) believes that there 

 is an actual invagination of entodermal cells (fig. 174A-B). More study is 

 needed to substantiate this view. 



Eventually, therefore, a secondary blastula arises which is composed of a 

 floor of entodermal cells, the hypoblast, closely associated with the yolk, and 

 an overlying layer or epiblast. The epiblast layer is formed of presumptive 

 epidermal, mesodermal, neural, and notochordal, organ-forming areas. The 

 essential arrangement of the presumptive organ-forming areas in the reptiles 

 is very similar to that described for the secondary avian blastula. The space 

 between the epiblast and hypoblast layers is the secondary blastocoelic space. 



VITELLOCYTES 

 CENTRAL BLASTODE R M 

 E N TO D 



Fig. 175. Early blastoderms of the prototherian mammal, Echidna. (A) Early blasto- 

 derm showing central mass of cells with peripherally placed vitellocytes. (B) Later 

 blastoderm. Central cells are expanding and the blastoderm is thinning out. Smaller 

 cells (in black) are migrating into surface layer. Vitellocytes have fused to form a 

 peripheral syncytial tissue. (C) Later blastoderm composed of a single layer of cells 

 of two kinds. The smaller cells in black represent potential entoderm cells. (D) Increase 

 of hypoblast cells and their migration into the archenteric space below to form a second 

 or hypoblast layer. 



