,Blostocoel 



^fe^ Gastrular ^^^^ 



Yolk 



Dorsal blasto- 

 pore lip 



Blastocoel 



Dorsal blasto- 

 pore lip 



germ ring ' 



^ Yolk plug 



Yolk plug 



Archenteron 



Medullary plate | Notochordal cells 



Archenter ^^^^^^^^^^^^.^^ 



)rsal blasto- 

 pore lip 



Yolk plug 



Blastocoel 



Archenteron 



Ventral blasto- 

 ip 



/ \ 



/ \ 



r Gastrular slit A 



Completion^ bridge 



\, Notochord 

 Archenteron ^ 



Mesoderm 



Yolk 



Blastocoel 



Yolk 



Involuted 

 mesoderm 



Gastrulation and early neurulation in the frog; (solid) ectoderm, (cellular) 

 endoderm and yolk, (striated) mesoderm, (circles) presumptive notochord 

 and mesoderm. 



125 



