no 



Germ ring 



GASTRULATION 



Epithelial ectoderm 



Blastocoel 



astocoel 



Side of the gray 

 crescent, blasto- 

 pore, onus, and 

 mesoderm forma- 

 tion 



Germ ring 



Dorsal 



Initial invagination 

 at dorsal lip 



Dorsal 



Margin of successive 

 involuted lip cells 



Portion of gray 

 •\ crescent 



Margin of germ ring 



Ectoderm Endoderm 



horda-mesoderm 



Dorsal blastopore lip 



Blastopore 



Posterior 

 Yolk plug 



Ventral blastopore lip 



esoderm 



Ventral 



Notochord 



Ectoderm 



Margin of ectoderm 



Anterior margin of 

 mesoderm 



Mesoderm 



Chorda -mesoderm 



Blastopore 



The process of gastrulation. (A) The blastula stage, prior to any gastrulation 

 movement. (B) Movement of the blastula cells preliminary to gastrulation. 



(Continued on facing page.) 



