GASTRULATION IN VARIOUS CHORDATA 



419 



fronj the upper or external opening to the archenteric space which lies below 

 the notochord and entoderm (fig. 200B, E). 



Posterior to the opening of the notochordal canal is the thickened primitive 

 plate (primitive streak), composed of converged presumptive mesodermal 

 cells (fig. 199). This converged mass of cells involutes to the inside along the 

 lateral borders of the notochordal canal and also posterior to this opening. 

 However, most of the mesoderm of the future body of the embryo apparently 

 passes inward with the notochordal material during the formation of the noto- 

 chordal canal, where it comes to lie on either side of the median notochordal 

 band between the ectoderm and the entoderm. These general relationships of 

 notochord, ectoderm, mesoderm, and entoderm are shown in figure 201A-H. 



The extent to which the original notochordal inpushing is developed varies 

 in different reptilian species. In lizards and snakes its development is more 

 pronounced than in turtles (cf. fig. 200A, D). 



During emboly, the presumptive neural plate and epidermal areas are 



N OTO C HORDAL CANAL 



^ PRIM 



INTERNAL OPENING OF 

 NOTOCHORDAL CANAL 



MESODERM 



BEGINNING 



NOTOCHORDAL 

 / CANAL 



: NT00E^^^<i'^=^^^^"'^^^.'^C. 



4i 



LATER NOTOCHORDAL CANAL 



Fig. 200. Sagittal section of reptilian blastoderms to show notochordal inpushing 

 (notochordal canal or pouch). (A) Section of early gastrulative procedure in Clemmys 

 leprosa. (After Pasteels, '36b, slightly modified.) (B) Original from slide, Chrysemys 

 picta. showing condition after notochordal canal has broken through into archenteric 

 space. (C) Notochordal canal of the lizard, Platyductylus. (D) Later stage of (C). 

 (E) After notochordal canal has broken through into archenteric space. (C-E, after 

 Will. 1892.) 



