GASTRULATION IN VARIOUS CHORDATA 



417 



E NTODERM 



NOTOCHORD 



MESODERM 



MESODERM 



Fig. 198. Anterior extension (migration) of the mesoderm from the blastoporal-lip 

 area after involution at the lip in the urodele, Plcurodeles. (A-C) Progressive inward 

 migration of the mantel of mesoderm, indicated by the white area stippled with coarse 

 dots. (A) Early gastrula. (B) Late gastrula. (C) Beginning neurula. 



As a result, the presumptive epidermal and neural plate areas literally engulf 

 the inwardly moving cells. 



3. Gastrulation in Reptiles 



a. Orientation 



The reptilian blastoderm, as gastrulation begins, is composed of an upper 

 epiblast and a lower hypoblast as indicated previously in Chapter 7 (fig. 

 174A-D). The formation of the hypoblast as a distinct layer proceeds in a 

 rapid fashion and immediately precedes the formation of a large notochordal 

 canal and subsequent cell migration inward. The two events of entodermal 

 layer (hypoblast) formation and the inward migration of notochordal and 

 mesodermal cells thus are closely and intimately correlated in reptiles. This 

 close relationship is true particularly of the turtle group. The upper layer or 

 epiblast of the reptilian blastoderm is a composite aggregation of presumptive 

 epidermal, neural, notochordal, and mesodermal cells (fig. 174E, F), arranged 

 in relation to the future, antero-posterior axis of the embryo. It is possible 

 that some entodermal material may be located superficially in the epiblast in 

 the turtle as gastrulation begins (Pasteels, '37a). 



b. Gastrulation 



Immediately following the formation of the hypoblast, the gastrulative phe- 

 nomena begin with a rather large inpushing or invagination involving the 

 notochordal, mesdoermal areas, particularly the pre-chordal plate and noto- 

 chordal areas. This invagination extends downward and forward toward the 

 hypoblast along the antero-posterior embryonic axis, and it produces a pouch- 

 like structure known variously as the notochordal canal, blastoporal canal, or 

 chordamesodermal canal (figs. 199A-C; 200A-C). The invaginated noto- 



