272 



THE HYDRA 



Fig. 132. ^Development of hj'dra. 



A, section showing two stages in formation of ovum within ovary. B, section of ecto- 

 derm showing early stage of ovary with several oocytes or female germ cells ig.c.) surrounded 

 by yolk-forming cells (y-f.c). C, four stages in development of the zygote, while still 

 attached to parent, showing one-cell stage with polar bodies (p.b.) and membrane (m), 

 two-cell stage, blastula with cleavage cavity (c.c). and later stage in which ectoderm and 

 endoderm have been differentiated. D, section of embryo soon after detachment from 

 parent showing cyst-like covering formed by further development of membrane. E, por- 

 tion of D showing inner and outer membranes of the cyst in relation to the ectoderm. 

 F, young hydra emerging from the cyst, c.c, cleavage cavity; ec, ectoderm; <», endo- 

 derm; g.c, germ cells; i.m., inner membrane; m, membrane; a.m., outer membrane; 

 ov, ovum; p.b., polar bodies; u-f.c., yolk-forming cells. (B and C, after Tannreuther.) 





