68 



REPRODUCTION AND LIFE HISTORY. 



which appears between ectoderm and endoderm, and into 

 which cells wander from these two layers. In the other 

 Metazoa, the middle layer may arise from a few primary 



FIG. 33. Life history of a coral, Monoxenia darwinii. 

 From Haeckel. 



A, B, Ovum. C, Division into two. I), Four-cell stage. E, P.las- 

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

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

 pleted gastrula, showing ectoderm, endoderm, and archenteron. 

 K, Free-swimming ciliated gastrula. 



