440 



GASTRULATION 



GERM RING 



EXTRA-EMBRYONIC 



BLASTODERM 



GERM RING 

 DORSAL LIP OF BLASTOPORE 



CLOSING BLASTOPORAL RING 



Fig. 211. Gastrulation in teleost fishes. (A-F after Wilson, 1889; G from Kerr, '19, 

 after Kopsch.) (A) Sea bass. 16 hours, embryonic shield becoming evident, marks 

 beginning of germ ring. (B) Germ ring well developed. Surface view of blastoderm 

 of 20 hours. (C) Side view of blastoderm shown in (B). (D) Side view, 25 hours. 

 (E) Surface view, 25 hours. (F) Side view, 31 hours. (G) Late gastrula of trout, 

 Sal mo fario. 



(3) 



is considerable variation among different species as to the degree of 

 separation of the entodermal layer; in the sea bass it appears to be 

 definitely separated, whereas in the trout it is reduced to a single layer 

 of entodermal cells lying below the notochord. The pre-chordal plate, 

 entoderm, and anterior notochord merge into a uniform mass below 

 the cranial end of the neural plate. 



Mesodermal convergence and involution. Along with the migration of 

 notochordal cells, the presumptive mesoderm converges posteriad to 

 the dorso-lateral lips of the blastopore, where it involutes and moves 



