66 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



central cavity the segmentation cavity and a hollow ball 

 of cells a blastosphere or blastula results. 



FIG. 13. Life history of a coral, Monoxenia Darivinii. 

 (From H/ECKKL.) 



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

 Blastula. F, Free swimming blastula with cilia. G, Section of 

 blastula. H, Beginning of invagination. I, Section of completed 

 gastrula showing ectoderm, endoderm, and archenteron. K, Free 

 swimming ciliated gastrula. 



