82 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



Other Metazoa, the middle layer may arise from a few 



Fk;. 40. — Life-history of a coral. Monoxenia darwinii. 

 — From Haeckcl. 



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

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

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

 pleted gastrula, showing ectoderm, endoderm, and archenteron. 

 K, Free-swimming ciliated gastrula. 



primary mesoblasts or cells which appear at an early stage 



