xxii DEVELOPMENT 247 



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



A ^ B _ C _ Dx^ni^N. E 



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

 niedea flexuosa (A-H) and Eudendriiim ramosum (I-M). 



A, oosperm. 



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

 u, 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 



