Medullary plate Notochord 

 Neural ectoderm / / Endodermal cells 

 Epithelial ectoderm / ///, Pigmented gut roof 



Archenteron 

 Gastrular slit 



Completion bridge 



Chorda-mesoderm 

 Dorsal blastopore lip 

 Blastocoel 

 Yolk plug 



Ventral blastopore lip 



Peristomial mesoderm 

 Gastrular silt 



Ectoderm 



Endoderm 



Notochord 

 A' 



Dorsal blastopore lip 



Yolk plug 

 Ventral blastopore lip 



Gastrulation and mesoderm formation — (Continued). (Top) Drawing to 

 be compared with top illustration on facing page. (Bottom) Schematic diagram 

 to show progression, internally, of the mesoderm. 



115 



