68 



REPRODUCTION AND LIFE HISTORY. 



and Ccelentera it is not a distinct layer except in Cteno- 

 phora, being usually represented by a gelatinous material 

 (mesoglcea), which appears between ectoderm and endoderm, 



FIG. 32. Life history of a coral, Monoxenia darwinit, 

 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, showing ectoderm, endoderm, and archenteron 

 K., Free-swimming ciliated gastrula. 



