INTRODUCTION 



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during gastrulation. However, the disappearance of the blastocoehc space 

 is more apparent than real. For, while most of the original blastocoelic space 

 is thus occupied and obliterated, a part of the original blastocoel does remain 

 as an extremely thin, potential area between the outside ectoderm and the 

 mesoderm-entoderm complex of cells. In flattened blastoderms, as in the 

 chick, the actual space between the ectoderm, mesoderm, and entoderm is 

 considerable (fig. 234E, F). To sum up: Though the blastocoelic space ap- 

 pears to disappear during the terminal phases of gastrulation, a residual or 

 potential space remains between the three germ layers, more pronounced in 

 some species than in others. This residual space gradually increases during 

 the tubulation processes of the major organ-forming areas. In doing so, it 

 permits not only the tubulation of these areas within the outside ectoderm, 

 but it allows important cell migrations to occur between the various body tubes. 



6. Primitive Circulatory Tubes or Blood Vessels 



Accompanying the tubulations of the epidermal, neural, entodermal, and 

 the two mesodermal areas on either side of the notochord, is the formation 



Fig. 221. Transverse sections through early neural fold embryo of the frog as shown 

 in fig. 220A and B. (A-J) Sections are indicated in fig. 220B by lines A-J, respectively. 

 Observe that the dorsal arching (dorsal upgrowth) movement of the dorsally situated 

 tissues accompanies neural tube formation. 



