418 



GASTRULATION 



AREA PELLUCIDA 



EMBRYONIC SHIELD 



NOTOCHORDAL CANAL 



OPENING 

 ^PRIMITIVE STREAK 

 AREA OPAC A 



BEGINNING OF 

 — HEAD FOLD 



NOTOCHORD 



^. 



INTERNAL OPENING 



F 



NOTOCHORDAL CANAL 



EXTERNAL 

 OF 



OPENING 



NOTOCHORDAL CANAL^ 

 PRIMITIVE STREAK 



c. 



Fig. 199. Surface views of blastoderm of the turtle, Chrysemys picta, during gastrula- 

 tion. Darkened area in the center shows the embryonic shield, the region of the noto- 

 chordal canal in the area of the primitive plate. (A) Young gastrula. External opening 

 of notochordal canal is wide. (B) Later gastrula. External opening of notochordal 

 canal is horseshoe-shaped; internal opening of canal is indicated by small crescentic light 

 area in front of externa! opening. (C) Very late gastrula. Notochord is indicated in 

 center; head fold is beginning at anterior extremity of blastoderm. 



chordal canal reposes upon the entoderm, and both fuse in the region of con- 

 tact (fig. 200C). The thin layer of cells in the area of fusion soon disappears, 

 leaving the antero-ventral end of the flattened notochordal canal exposed 

 to the archenteric space below. After some reorganization, the notochord 

 app-'^ars as a band, extending antero-posteriorly in the median line, associated 

 with the entoderm on either side (fig. 201B-G). However, at the extreme 

 anterior end of the gastrula, the notochordal material, together with the ento- 

 derm and to some extent the overlying ectoderm, presents a fused condition. 

 Within this area the pre-chordal plate or anterior portion of the head organizer 

 is located. In this general region of the embryo, foregut, brain, and other head 

 structures eventually arise (fig. 199C). The original, relatively large, noto- 

 chordal invagination soon becomes a small canal which extends cranio-ventrally 



