GASTRULATION IN VARIOUS CHORDATA 



437 



the blastoderm (fig. 211 A, B). This posterior prominence of the germ ring 

 forms the embryonic shield. 



As gastrulation begins, the entodermal cells of the primary hypoblast at the 

 caudal edge of the embryonic shield stream forward below the epiblast toward 

 the anterior end of the blastodisc (figs. 210A, D). Coincident with this for- 

 ward movement of the primary hypoblast, a small, crescent-shaped opening 



EMBRYONIC SHIELD 



NEURAL PLATE MATERIAL 



^, r-Ti'Ejej-'-E;::^.^ 



GERM RING 



■ E M B RVON I C 



Fig. 210. Gastrulation in teleost fishes. (A) Sagittal section of early gastrula. (Modi- 

 fied slightly from Wilson, 1889.) (B) Midsagittal section through late teleost gastrula. 

 The dorsal and ventral lips of the blastopore are shown approaching each other. (Modi- 

 fied slightly from Wilson, 1889.) (C) Beginning gastrula of early blastoderm of brook 

 trout, Salvelinus. Observe inward (forward) migration of primary hypoblast cells and 

 thickened mass of cells which arises at posterior margin. (After Sumner, '03.) (D) 

 Later stage in gastrulation of brook trout. (After Sumner, '03.) (E) Transverse section 

 of late gastrula of brook trout, showing the three germ layers. (After Sumner, '03.) 

 (F) Transverse section through late gastrula of sea bass. (After Wilson, 1889.) (G) 

 Midsagittal section through closing blastopore of sea bass. (After Wilson, 1889.) (H) 

 Longitudinal section through late gastrula of the brook trout. (After Sumner, '03.) 



