HYDKA 



321 



develops at the expense of the other oogoiiia, which are engulfed 

 bodily and used for food. This one cell grows rapidly, and when 

 mature it fills the ovary. Fertilization is accomplished by the en- 

 trance of spermatozoa through a rupture in the overlying ectoderm 

 and cross-fertilization usually prevails. A single sperm unites with 

 the mature ovum, and this zygote undergoes the total and equal 

 divisions of cleavage here in place. The process continues until a 

 hollow hlastula of many cells is formed. Then follows the formation 

 of the gastnda by a shedding of cells into the cavity (blastocoele) 



Fig-. 115. — Development of hydra. 1, Fertilized ovum ; 2, two-cell stage ; S, 

 blastula stage; 4, gastrula, showing ectoderm (ec) and endoderm {en) ; cc, cleavage 

 cavity (blastocoele); m, cyst; p.b., polar bodies. (After Tanreuther, Biological 

 Bulletin, Vol. 14.) .^ 



from the inside of the original layer of cells. These new cells on the 

 inside become organized as an endoderm layer, while the original 

 outer layer is now known as ectoderm. Further changes involve the 

 secretion of the thin mesogloea which seals the two layers together. 

 In the meanwhile a shell is produced about the outer surface of the 

 embryo, and this encysted body falls from the parent to the bottom. 

 If conditions are favorable for development, :t increases in length 

 within the cyst; when it has attained some size it breai^s out, after 



