XXII DEVELOPMENT 247 



merit, a dilatation is formed a short distance from the free or 

 B c 



Fig. 57. — Stages in the development of two hydroid polypes, Lao- 

 medea flexttosa (a-h) and Eudendrium ramosum (I-m). 



A, oosperm. 



B, two-celled, and c, four-celled stage. 

 D, E, polyplast. 



F, G, formation of planula by differentiation of ectoderm and 

 endoderm. 



In A-G the embryo is embedded in the maternal tissues. 



H, free swimming planula, showing ciliated ectoderm, and endoderm 

 enclosing a narrow enteric cavity. 



I, planula, after loss of its cilia, about to affix itself. 



K, the same after fixation. 



L, Hydra-like stage, still enclosed in cuticle. 



M, the same after rupture of the cuticle and liberation of the tentacles. 

 (After Allman.) 



distal end, and a thin cuticle is secreted from the whole 

 surface of the ectoderm (k). From the dilated portion 



