GASTRULATION IN VARIOUS CHORDATA 



409 



in a manner similar to the pre-chordal and notochordal cells; rather, upon 

 reaching the edge of the blastopore, it involutes over the lip, then insinuates 

 itself between the inside entoderm and the external surface layer of cells, and, 

 in this position, passes inward and forward between the entoderm and the 

 external layer of cells (figs. 19 IH; 198 A). 



YOLK PLUG 



Fig. 191. Migration of the presumptive organ-forming areas of the blastula during 

 gastrulation in the amphibia (with reference particularly to the frog). (See fig. 192.) 

 (A) Late blastula, sagittal section through midplane of future embryo. (B-F) Observe 

 processes of epiboly and emboly. In epiboly, the black (neural) and white (epidermal) 

 areas become extended and gradually envelop (fig. 192A) the inward moving notochord, 

 entoderm, and mesoderm. The processes concerned with emboly bring about the inward 

 migration of the latter presumptive areas. (G) Late gastrular condition, with neural 

 area and upper portion of the epidermal area removed to show relationships of the middle 

 germ layer of chordamesoderm. (H) Horizontal section of middle gastrular condition, 

 showing involution of mesoderm between entoderm and ectoderm. (I) Late gastrula, 

 horizontal section, showing yolk plug, mesoderm, and final engulfment of blastocoelic 

 space by entoderm. 



