GASTRULA. 



65 



establishment of the two primary germinal layers, the outer 

 ectoderm and the inner endoderm, or the epiblast and the 

 hypoblast. 



Fig. 30. — Life history of a coral, Monoxenia Darwinii. 

 — From Haeckel. 



A, B, Ovum. C, Division into two. D, Four-cell stage. E,_ Bias- 

 tula. F, Free-swimming blastula with cilia. G,_ Section of 

 blastula. H, Beginning of invagination. I, Section of com- 

 pleted gastrula, snowing ectoderm, endoderm, and archenteron- 

 K, Free-swimming ciliated gastrula. 



