Medullary plate 

 (neural ectoderm) 



Epithelial ectoderm 

 Notochord 



Endoderm 



Completion 

 bridge 



Blastocoel 



Chorda-mesoderm 



Dorsal blasto- 

 pore lip 



rS#-Yolk plug 



Ventral blasto- 

 pore lip 



Gastrular slit 

 Peristomial mesoderm 



Endoderm ^vtf^*""^ 3k 



Archenteron Notochord^^;^' ' 



Ectoderm v^.x„.V 



Blastocoel Chorda- mesoderm^/^;.>^^ 

 Completion bridge 





'r'MXi ^ 





Yolk plug,j5 



. -/. 'J^'>;s^T ^*<''-*^^«K[»ia^"^esoderm 



^^^^:!r^' *^W^'tS£9S^'¥/&^SSr- Ectoderm 



Gastrular slit 



Gastrulation and mesoderm formation. {Top') Photograph of sagittal section 

 through the blastopore. (Bottom) Enlarged photograph similar to above illus- 

 tration. Note the continuity of the ventral ectoderm and peristomial mesoderm. 



{Continued on facing page.) 



114 



