420 



GASTRULATION 



INTERNAL OPENING OF 



NOTOCHORDIL CANAL 



EXTERNAL OPENING OF 



NOT OCHORDAL CANAL 



Fig. 201. Transverse sections of the late turtle gastrula as indicated by lines in fig. 199C. 



elongated antero-posteriorly by the forces of epiboly. Meanwhile, the external 

 opening of the notochordal canal changes in shape and together with the 

 primitive plate moves caudally (fig. 199). As gastrulation draws to a close, 

 the neural plate area begins to fold inward, initiating the formation of the 

 neural tube. 



4. Gastrulation in the Chick 

 a. Orientation 



As described in Chapter 7, a two-layered blastoderm (blastula) composed 

 of an epiblast and a hypoblast is present, with the hypoblast more complete 

 at the posterior end of the blastoderm than at its extreme anterior and antero- 

 lateral margins (figs. 171 A; 202A). The epiblast over the posterior half of 

 the blastoderm is composed of presumptive notochordal and mesodermal cells, 

 and anteriorly in the epiblast are found the presumptive epidermal and neural 

 areas (figs. 173A; 202A). 



b. Gastrulative Changes 



1) Development of Primitive Streak as Viewed from the Surface of Stained 

 Blastoderms. The formation of the primitive streak is a progressive affair. 

 Figure 170 pictures a pre-streak blastoderm, and it is to be observed that the 

 ectodermal layer below the epiblast is present as an irregular area in the 



